What is it called when the subject (being you) is automatically understood?

Like...

Go shut the door.

the subject is understood to be..you.

what is that called?

"Go shut the door." is an imperative sentence because it gives a command. Most imperative sentences have the understood "you" as the subject of the verb.

=) Thank you.

You're welcome! The concept you're referring to is called an implied subject or an implied pronoun. In the sentence "Go shut the door," the subject "you" is understood or implied, even though it is not explicitly stated. This is common in imperative sentences where the subject of the command is assumed to be the person being addressed. So, in this case, "you" is the implied subject of the verb "go" and the command is directed at the person being spoken to.