Canary release is a technique to reduce the risk of introducing a new software version in production by slowly rolling out the change to a small subset of users before rolling it out to the entire infrastructure and making it available to everybody. Canary testing is recommended as good practice when it comes to agile delivery aimed to release software in small chunks.
The name for this technique originates from miners who would carry a canary in a cage down the coal mines. If toxic gases leaked into the mine, it would kill the canary before killing the miners. A canary release provides a similar form of early warning for potential problems before impacting your entire production infrastructure or user base.
Interesting example of ideas from mining adopted in software.