Let's

[Unregistered]

What is the meaning (not function) 0f the sentence "let's go to the theatre."

[David L.]

"let's" is used in casual conversation and indicates that the speaker is making a suggestion.
Another way of saying this is:"Why don't we go to the theatre?" Here, the context, the conversation, and the speaker's tone of voice would indicate that the speaker is not puzzled as to why they are not going to the theatre and asking for a reason, but again, that he is making a suggestion.