When someone asks you a question, and you're not sure of the answer, you can say "It depends." This phrase means that you can't answer until you know more information. In the example above, you can't answer the question, "Do you want some breakfast?" until you know what your mother is cooking.
You tell what information you need to know in order to make the decision using "depends on ___":
It depends on what you're making.
You can also just say "It depends." and then ask a question:
A: Do you want to come?
B: It depends. Who else is going?