Pitch a tent is a slang phrase meaning “have an erection strong enough to lift some type of fabric.” The phrase is typically used to describe a man having an erection while wearing trousers or shorts, or while lying down underneath sheets or a blanket.
The term pitch a tent originated because an erect penis standing straight up underneath some kind of fabric can appear something like a tent pole holding up a tent. Have a tent pole or get a tent pole is a variation of the term pitch a tent.