A canary deployment is used in software deployment to test new releases of an application or service by making it available to a small subset of users or servers instead of releasing it directly to production. This group of users serves as testers to determine any potential issues with the released version.
The name came from canary birds used in coal mines as detectors of gases as they were highly sensitive to them. In this way, when a new app version is released to a subset of users instead of a wide audience, developers can address potential issues easier and with less impact.