The imperative mood, whose name derives from the Latin word for I order, refers to sentences whose intent is to instruct the reader or listener to perform a given action. Examples:

'Shut the door, Alec; there's a draught!' (actually, only the first clause there is imperative.)
Consider yourself at home.

As ever with moods, the mood is conveyed by a combination of the verb form employed and the structure of the sentence. Compare indicative mood and subjunctive mood.