A champion is the person inside your prospect's company who believes in your solution and actively sells it internally on your behalf. They are not necessarily the economic buyer or the decision maker; they are the advocate who pushes your proposal forward, answers objections in meetings you are not in, and connects you to the right people. A real champion also smooths the handoff between every stage of the deal. Finding a champion early in discovery is often the difference between a deal that moves and one that stalls.
In plain English
A champion is your inside ally. You cannot be in every room where your deal is discussed, but a champion is. When you have a real champion, your solution gets defended when you are not there; when you do not, your demo is forgotten a week later.
Why it matters to you
In any account executive or closing motion, your job is to identify who is excited enough to advocate for you. Ask "who stands to gain if this gets approved?" A deal with a champion is worth far more than the same deal without one. In interviews, the ability to describe how you found and armed a champion is a mark of a seasoned seller.