policy name: repository_dismiss_stale_reviews
This security control prevents merging code that was approved but later on changed. Turning it on ensures new changes are required to be reviewed again. This setting is part of the Merge request approval settings, and hardens the code-review process. If turned off - a developer can change the code after approval, and push code that is different from the one that was previously allowed.
Buggy or insecure code may be committed after approval and will reach the main branch without review. Alternatively, an attacker can attempt a just-in-time attack to introduce dangerous code just before merge.