OEM means Original Equipment Manufacturer.
The original equipment manufacturer is the company that made the part (in this case the key) that originally was supplied with the vehicle. Sometimes it's not easy to determine if a part is OEM, because aftermarket companies may have gotten permission to make reproductions of the original part.
We should note that just because a part is OEM doesn't guarantee that the part is identical to that originally supplied with your car. For example, Briggs & Stratton (now Strattec), an OEM manufacturer for many cars, is still manufacturing keys today, many of which will fit cars made 75 years ago. However, they aren't labeled anything like the original keys were.
Generally, when we refer to a key as OEM, we mean that it's identical to the original part.