A flaky test gives different results even when the project has not changed: sometimes it passes and sometimes it fails. It is like a doorbell that works only on some presses.
One cause might be a test that depends on timing or on a service that is sometimes slow. The changing result makes it harder for a team to know whether a new code change really caused a problem.
Flaky tests should be investigated rather than ignored. A test that cannot give a dependable signal may hide real failures or waste time with false alarms.