My understanding is that a beam in this game is a structural component of the vehicles (used in the crash physics) but what does the NG mean? I feel that's something I should know, given I have over 100 hours of play time on this game!
Next Generation perhaps? Since a bunch of the devs came from Rigs of Rods, and the physics simulation is greatly improved from that beam-based game.
I can confirm that BeamNG stands for Beam Next Generation, as the physics engine in ROR was called Beam.