FMEA works asset by asset, listing every plausible failure mode — a seal that leaks, a sensor that drifts, a motor that overheats — then scoring each on severity, likelihood of occurrence, and how easily it would be caught before it causes a problem. Multiplying those three scores gives a risk priority number that ranks where attention should go first.
It's the analytical groundwork RCM is built on — you can't decide the right maintenance strategy for an asset's failure modes until FMEA has actually enumerated what they are and how much each one matters.