"Rock the Casbah" is a song from the 1980s that was made popular by a band called The Clash. The song tells a story about a king in a faraway land who doesn't like his people listening to rock and roll music. The king wants his subjects to stick to traditional music, but the people secretly love rock music and want to listen to it.
The phrase "rock the casbah" means to rebel against the rules and do something different or exciting. In this case, it means the people are going to secretly play their rock music and have a good time, even though the king doesn't want them to.
So, when people say "let's rock the casbah", they mean they want to break the rules, rebel, and do something fun and exciting. It's like saying "let's have a good time and forget about the rules for a little bit."