Q: What language does a billboard speak?
A: Sign language!
Explanation: A billboard is one of those really big advertisements found along roads and on top of buildings; a billboard is a type of sign. This joke plays on the word “sign” which can mean a public, printed announcement that is usually large (like a street sign, a stop sign, a road sign, the name of a store on its facade, …) or it can be the first part of “sign language.” Sign language is a language used by hearing impaired people to communicate. So a billboard (a sign) speaks sign language because it is a sign!
As you work on learning English or learning jokes take some time to learn sign language!