An imperative sentence is a sentence that always addresses the second person (whether plural or singular) so the subject is implied most of the time (it is not directly stated) and is used to give a command, an instruction, a piece of advice, or to make a request. Here are some examples (Imperative sentences underlined):
Please, stand up. (A request)
Do not look at me like that! (An order)
Please, listen to what I say, don't be so grumpy, it will not take anywhere. (Advice)