Plain English Meaning
The right-hand one of the two magnetos that fire the spark plugs. When the ignition switch is set to R, only that magneto is firing the engine.
Full Definition
In the context of aircraft ignition systems, RIGHT refers to the right magneto — one of the two independent magnetos that supply electrical current to one set of spark plugs in each cylinder. The ignition switch position labeled R isolates the engine to run on the right magneto only, typically used during the magneto check on runup.
Context Anchor
Seen on ignition switches and during the engine run-up when checking LEFT, RIGHT, and BOTH ignition operation.