what's the difference?

idiom

1

—used to ask how one thing is different from another

"I like this one a lot more than that one." "Why? What's the difference (between them)?"

2

or what difference does it/that make?

—used to ask why something is important or to suggest that something is not important

What's the difference whether I go or not?

What difference does it make if I go with you?