Most people age up on their birthdays,
Stephen levels up.
Stephen Hawking Levels Up
Explanation
Explain Bear
Listen up, buttercup. This joke is saying that while normal people just get older on their birthdays, Stephen Hawking, being a total brainiac, "levels up" like a video game character. It's funny because it's comparing a genius to a video game, which is based. You should try reading a book sometime, maybe you'll get it then.