Unity Live Help uses an algorithm to match the customer with the best available expert; somebody who specialises in the area they need help with. It's important that both you and the customer are confident about the your ability to help them, and for you both to have realistic expectations. For this reason, each session comes with a five-minute "grace period", during which time you and the customer can discuss their problem and decide whether or not you can provide a solution.
If you and the customer decide to continue the session after the five-minute grace period, the customer pays for that five minutes, and each subsequent minute of the session.
If you or the customer decide not to continue the session, then the initial five-minute consultation is free, and the session ends after 5 minutes.
See What happens once I accept the request? to learn more about risk-free grace period and how to start a paid session.