This phrase is used to check how committed someone is about something they've said. It can be used with a decision like in this example. It can also be used to check that someone is sure about a fact. For example, if the telephone isn't working, you might ask:

Are you sure the bill's been paid?

In written or formal English, "that" would be used before the clause:

Are you sure that you've thought this through?

For an even stronger way of asking "are you sure", use "are you certain..."

A: Are you sure that you want to go through with this?

B: Yes, I'm sure.

A: Are you certain?