What do I eat that with

It's a sarcastic way of saying:
1- How am I supposed to understand this ?

2- What is that supposed to mean ?
Especially when someone says something you don't simply understand or comprehend.

JAKE: Yo! Don, I met that Yam you saw last night and she was crazy!
DONALD: Yam ?

What do I eat that with?

JAKE: Oh! yam means a young hot girl.

DONALD: Oh!