In short
OEE is a metric that summarises production efficiency in a single number, calculated by multiplying an asset's availability, performance and quality rates.
Availability measures how much the machine runs during planned time, performance how close it gets to ideal speed while running, and quality how much of the output is right first time.
The power of OEE lies in how it classifies losses: downtime, speed losses and quality losses become visible separately. Reliable OEE depends on capturing downtime reasons accurately and automatically.