Chromium’s “feature flag” system also used by WebRTC. The Google variant is called “Finch”.
It allows a gradual rollout of new features to a certain percentage of users without changes to the installed (or deployed) software. In Chrome/WebRTC this is used to enable big features, e.g. the switch from usrsctp to dcsctp.
It also allows experimentation and tuning of parameters, most recently tuning automatic gain control.
For developers WebRTC field trials can cause headaches. Since these are internal to Google, testing your version might not reveal behavior or bugs that rely on a specific field trial being set, and there is no easy way to pick and choose which field trials to run. If you happen to have an issue that reproduces on one system but not the other a field trial might be involved.