It's mean to tell someone you can't stand them.
When Marty asked Liz to go to the dance with him, Liz was taken aback because she didn't even like Marty.
Liz wants to turn down Marty's invitation to the dance.
Liz tells Marty that she already has a date but that she's flattered he asked her.
Liz tells Marty that she can't stand him and that she won't go to the dance with him.
Marty is a little crestfallen, but he snaps out of it very quickly.
Marty's confidence is crushed, and he never asks out another girl ever again.