Exploratory testing
Also called: exploratory session, session-based testing
Exploratory testing is testing where design and execution happen together: the tester learns the product and decides the next check from what the last one showed.
In exploratory testing the tester is not following a script. They hold a charter, such as what happens to a booking when the connection drops, and follow what they find. It is the fastest way to learn how a changed area behaves, and it finds the problems nobody thought to write a case for.
It is not unstructured clicking. A session has a charter, a timebox and notes, and its findings are written up as defects, questions or new checks.
I spend exploratory time around whatever a release changed, and leave the stable core to repeatable checks. Pairing during a session, with a developer or a newer tester, makes it more useful still.