A candidate is someone running for political office — or anyone being considered for a position or opportunity of some sort.
When you hear about candidates, it's usually in discussions of politics. People are always wondering, "Who will be the candidates in the next Presidential election?" Every President, and every office-holder, was once a candidate. When you're a candidate, you raise money, hold rallies, make campaign promises, and do everything you can to win. You can also say someone is a candidate for a job, prize, and other things. Someone could jokingly say, "He's a candidate to be my next husband."