This book is a collection of Tome stuff.
There's more Tome stuff at this point than can possibly go into a single book. Literally, "The Tome Universe" as a whole has even become self-contradictory in some places because different authors have each made their own versions of how things should be. This is a best effort attempt to patch everything together, but of course nobody's perfect.
The System Reference Document describes an RPG ruleset that is pretty good, but with a lot of cracks that show up the more you play. The fact that fighter-types and caster-types don't gain power at the same rate is the biggest, and easiest, problem to point to. The way that magic items work (or rather, don't) at later levels is another. So, many groups try to fix the Fighter/Wizard imbalance by making the Wizard (and other casters) weaker, until they're as weak as the fighter.
The problem is that there's a lot of game involved (like 700 pages or so, at least), which makes adjusting all of it into some sort of "fix" a monumental task. There's tons of spells, tons of items, tons of monsters... and when you look at it closely, it turns out that most caster-types are lined up just right with all the monsters (especially since many monsters cast the same spells at mid to high levels). Really, it's just the fighter-types that are out of alignment.
So two guys decided to write a series of forum posts that did the opposite of the "make casters weaker" style fix people normally try. Instead of dragging strong options down, they made fighter-types and other "weak" options much better; enough that they became very competitive with the strong options. There were four big posts put out at first, the Tome of Necromancy, the Tome of Fiends, the Dungeonomicon, and Races of War; This series came to be known as "The Tomes". They had pretty good balance, and lots of people liked the fluff too. Soon other people were posting Tome feats, and Tome classes, and other rules and character options to fill out the ranks of a Tome game.
If you're really new you might not even know this much. A roleplaying game is where you pretend to be a character in some sort of imaginary world. A table-top roleplaying game is played in a group (often with a table, but sometimes over IM or Skype) where one player is the "Game Master" (GM) who describes the world, and everyone else controls a "Player Character" (PC) who acts within the world. Players describe the actions of their character, the GM describes the results, and then the players pick more actions and hear about more results. Usually for a few hours at a time, known as a "session". There's generally a single GM and 3 to 6 PCs.
Because the term Game Master sometimes gives people ideas about absolute authority and such, which can make people into jerks, the GM is sometimes called the MC instead. MC usually stands for "Master of Ceremonies", but within this game it also stands for "Mister Cavern" or "Miss Cavern", because the MC "plays" in some sense the role of the uncaring world that the player characters live within. It's a less authoritarian sounding title.
Of course, you and your friends can play pretend all on your own without any rules, that's what Cops and Robbers is. However, this game provides rules for numbers to be attatched to characters and for the rolling of dice based on those numbers when results are uncertain, so that an element of impartialness can be attained. The MC is still responsible for what the players discover, but they decide how to respond, and if they decide to take on challenges the dice are the ultimate arbiters of success or failure, not purely the MC's whims. As much as possible, the MC should attempt to be impartial when running the game. Nobody "wins" if all the party dies, that's just the end of the story. The winning and losing is in how entertaining the story was to experience.
The rules cannot cover everything, and so the MC must also handle the common case where a player tries something that's not explicitly in the rules. Usually the group can come to an agreement as to what should happen, based on existing rules.
Tome is a fantasy game with knights and wizards, and has many of the storts of things that you'd expect, but it's usually full of a lot more magic than stories like Lord of the Rings or Conan the Barbarian. It covers a bigger power scale than many books and movies tend to. Characters can go from crap covered 1st level nobody, lucky to defeat a single orc 1-on-1 in a fight, up to a powerful force to be reckoned with, able to sway the fates of an entire planet with a single order. Naturally, this wide of a range can cater to a lot of people. Most people tend to play in the 4th - 10th level range most of the time, which is around the Lord of the Rings, Greek Myth, and Nordic Myth type of range.
Generally, the setting is some sort of magical world that's either recently passed through some sort of calamity, or is about to pass through some sort of calamity. Organized civilization, with law and order and such, isn't always weak, but is usually limited to a relatively small area. Most places are just small towns, farming communities, and wilderness. The roads are often dangerous to travel, and everyone tends to raid everyone else as often as they can get away with it.
Player Characters travel in a group, commonly known as a "party" (short for "war party"). The party goes about doing whatever it is that they do, usually something to do with fighting people and taking all of their stuff once they're dead. Often heroes, but usually in a more ancient world "might makes right" sort of way.
Unlike the main characters of many fantasy stories, player characters in Tome often have regular access to supernatural powers, and magic is not particularly rare. It's generally not fully "understood", in a scientific sense, but it's often around, and even most common folk have seen some sort of magic at least once in their life.