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?