its very difficult to understand the crfsuite features

[paktiger]

anyone can assist me what is the mean of these features? And how we can customize it or add more features
features = {
'bias': 1.0,
'word.lower()': word.lower(),
'word[-3:]': word[-3:],
'word[-2:]': word[-2:],
'word.isupper()': word.isupper(),
'word.istitle()': word.istitle(),
'word.isdigit()': word.isdigit(),
'postag': postag,
'postag[:2]': postag[:2],
}
if i > 0:
word1 = sent[i-1][0]
postag1 = sent[i-1][1]
features.update({
'-1:word.lower()': word1.lower(),
'-1:word.istitle()': word1.istitle(),
'-1:word.isupper()': word1.isupper(),
'-1:postag': postag1,
'-1:postag[:2]': postag1[:2],
})
else:
features['BOS'] = True

if i < len(sent)-1:
    word1 = sent[i+1][0]
    postag1 = sent[i+1][1]
    features.update({
        '+1:word.lower()': word1.lower(),
        '+1:word.istitle()': word1.istitle(),
        '+1:word.isupper()': word1.isupper(),
        '+1:postag': postag1,
        '+1:postag[:2]': postag1[:2],
    })
else:
    features['EOS'] = True