This is an imperative sentence.  It is an imperative sentence because it is giving a command.  You can almost imagine the word "You" in front of the sentence giving a command for someone to do something.  Command meaning imperative or of importance, and thus the sentence is an imperative sentence, as if giving an order to be carried out.

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It is an imperative sentence as the sentence is expressing a command.
Definition of an imperative sentence:
The sentence which expresses command,request or advice is called imperative sentence.