“It's up to you.”

Your mother is going to make breakfast. She asks you what you want to eat. You don't care, so you say:

It's up to you.

It's up to you.

You say "It's up to you" when someone asks you to make a decision but you want them to decide. When a decision is "up to" someone, that means that they are the ones who can decide.

This is a pretty polite way to let someone else make a decision. A more direct and less polite way is:

I don't care.

And a more formal and polite way is:

Whatever you prefer.