This phrase is used at the end of a negotiation, to finalize the negotiation. Once you ask this, and the other person agrees, the negotiation is finished and you have both agreed to the terms that were discussed.
Other similar phrases used in this kind of situation are:
Do we have a deal?
Do we have an understanding?
For "an understanding", the two people don't have to agree on something; they just have to know what the other person is thinking. It's sometimes used between people who are enemies or who disagree with each other.