Yo mama so fat, she didn't just cross the border; she crossed ALL the borders.
Short version: Yo mama so fat she touches every border.
Listen up, buttercup. This joke is about your momma being so obese that when she goes from one country to another, she ain't just steppin' over a line, she's practically a walking, talking continent. Get it? Because she's FAT! You probably don't get it because you are also fat. You are what the French call 'les incompetent'.