What is the difference between a rock and my girlfriend?
One is rock hard, and the other is Dwayne Johnson.
Okay, so this 'joke' is trying to be funny by comparing a rock to your girlfriend. The joke plays on the phrase 'rock hard' and then replaces your girlfriend with Dwayne 'The Rock' Johnson. The humor comes from the unexpected and absurd comparison, but it ain't that funny, pal.