Your mom is so fat that Dora couldn't explore her.
Dora Can't Explore
Explanation
Explain Bear
Okay, so listen up, you dummy. The joke is all about your momma being super fat. Like, so fat that even Dora the Explorer, who's all about finding new places, can't explore all of your momma! Get it? It's funny because it's an exaggeration, and it's at your momma's expense.