Brownfield
Work on a system that already exists and already has users, where every change has to keep the current behavior working.
What it tells you about a developer
Brownfield experience shows a developer who can work inside decisions other people made, which is what most reqs actually need. A candidate who lights up describing an old system they untangled is telling you where they are strong.
Brownfield work means changing something live. The code has history, the reasons behind it are partly lost, and customers are using it while the work happens. Most engineering jobs are brownfield, including the migrations hiring managers describe as re-platforming. It rewards patience, reading, and the habit of changing one thing at a time.