An imperative verb gives a command. An imperative sentence, likewise, is a command in distinction from a declarative sentence which is a statement.
The subject of an imperative verb is rarely stated. It is an implied “you” e.g.
Go and get the mail.
(implied) – (you) go and get the mail.
The imperative form is simply the prefix form with the prefix lopped off. See KHW chp. 18, this video or this chart.