[Rules] Add Rules Concepts page

This covers 1 and 1a of the table at
openhab#1855.

Signed-off-by: Florian Hotze <[email protected]>

rules/concepts.md

@@ -1,3 +1,4 @@
+<!---
 # Cover generic rules concepts
 
 Pull stuff from the Getting Started Tutorial.
@@ -22,4 +23,59 @@ formerly implicit variables
 How to find and install.
 How to write will be on a later page.
 
-### Comprehensive Examples
+### Comprehensive Examples
+-->
+
+# Rules Concepts
+
+Thus far we've connected openHAB to devices through Things, modeled the devices with Items, discussed persistence and how to build the display to control your home.
+But all of that amounts to home control, not automation.
+To create home automation we need to define rules.
+
+{::options toc_levels="2..4"/}
+
+- TOC
+{:toc}
+
+## What Are Rules
+
+You can think of rules as routines or behaviours for your smart home.
+
+Just like you stand up in the morning and have your breakfast, your smart home can have a routine to open the blinds when the sun rises.
+Another routine would be to turn all lights off when you leave home: openHAB can do that for you, no need to open your app and switch of the lights manually.
+You want to hear your favorite music when you arrive back at home? No problem, openHAB can do that for you.
+But rules can also remind you of things: you opened the window hours ago and forgot that is is open? openHAB can sent a notification to your phone.
+
+To summarize in an universal principle: *When __a__ happens, than do __b__*.
+For many cases, it can be practical to extend that to *When __a__ happens, than do __b__ but only if __c__*.
+
+To take some examples from above, you can have openHAB to only turn on your favorite music when you arrive at home if it is not your kids bedtime.
+You do not want openHAB to open your blinds on the weekend because you want to sleep a little bit longer? No problem, *but only if* helps.
+
+## Parts of a Rule
+
+To work with the *When __a__ happens, than do __b__ (but only if __c__)* principle, openHAB rules consist of three parts:
+
+| Name        | MainUI Section | Purpose                                                                                |
+|-------------|----------------|----------------------------------------------------------------------------------------|
+| `Trigger`   | When           | The *When __a__ happens* part: It causes the rule run when the defined event happens.    |
+| `Action`    | Then           | The *than do __b__* part: What should be done when the rule runs?                      |
+| `Condition` | But Only If    | The *but only if __c__* part: Which condition has to be met that the rule really runs? |
+
+The __a__, that causes the rule to run, is an so-called event in openHAB.
+
+::: tip A few words about events
+openHAB is an event-driven system.
+That means, that everything that happens with Items and Things is an event and some parts of openHAB listen to these events.
+When your kitchen light (Item) turns off, this is an event. And when openHAB lost connection to your kitchen's lightbulb (Thing) that is an event too.
+The UIs will see the Item event that your kitchen light turned off and update their state accordingly.
+And a rule can trigger on the Thing event "lost connection to the lightbulb" and perform an action, e.g. sent a notification to your phone.
+:::
+
+Any single rule is not required to have all of these (although a rule without an action is not very sensible).
+
+A rule that should always run when triggered will have 0 conditions.
+A rule that should respond to several different events can have multiple different triggers.
+Even though it might not seem sensible first, a rule with no trigger (that means it would never run, except manually) can still be useful: you can use it as an reusable action to call from other rules.
+These rules are called *Script Actions* in openHAB, more about them later.
+