Canary releases roll out new software versions to a subset of users as an initial test. Both the original version and the new verison are deployed simultaneously to production. The subset can be chosen by different strategies, including just random traffic. Also known as a phased rollout or incremental rollout.
As you gain more confidence, you can slowly increase the subset percentage, until eventually all users are on the newer version.
This strategy gives you a way of monitoring for errors while only affecting a small amount of users. If any errors occur, the percentage can be moved back down to 0%, so all users are back on a known working version.