What does the expression “to be on the beam” mean and Where does it come from?

By this very modern expression “to be on the beam” we mean “to be on the right course”; hence, right, accurate, correct.

It comes from the use of directional radio beacons established along lines of airplane travel.

A continuous pulsating note emitted from the beacon is heard at its greatest intensity by an aviator when he is directly upon the course along which he should be traveling, fading off to dimness if he departs from the course to one side or the other.