A quality gate is a check that work must pass before it can move to the next step. For example, a project might require its tests to pass before a change can be added to the main version.
Imagine a checkpoint before a bridge: someone checks that the required safety items are ready before letting the vehicle cross.
The gate is only useful if its conditions match what matters. Passing a test does not prove that everything is perfect, but it can catch known problems before they reach users.