Grammatical mood describes the relationship of a verb with reality and intent. For example, in English, the difference between «I insist that the chairman resign» (subjunctive), «the chairman resigns» (indicative) and «resign!» (imperative) is a difference of mood.
The indicative mood is used for factual statements and opinions or to make inquiries. Most of the statements you make or you read will be in the indicative mood. Examples: «Paul is reading a book», «Joe picks up the boxes».
As opposed to the indicative mood, the subjunctive mood signals «non-actual» or «non-factual» meanings. It is used to express condition, hypothesis, contingency, potentiality, possibility, uncertainty, prediction, obligation, desire, etc., rather than to state an actual fact. English has had a subjunctive mood but most of the functions of the old subjunctive have been taken over by auxiliary verbs like «may» and «should», and the subjunctive survives only in very limited situations e.g. «We insist that he do the job properly», «God save the Queen».
The imperative mood expresses direct commands or requests. It's also used to signal a prohibition, permission or any other kind of exhortation. In Persian, the imperative mood forms from the subjunctive mood. The only difference is that, the second person singular doesn't take its associated conjugative enclitic. In English, the imperative mood is formed simply by using the verb's plain form (e.g. go!) and the subject of the sentence can only be «you» (the 2nd person). In contrast, in Persian all persons have an imperative form and therefore, the imperative mood is not limited to the 2nd person.