The rating bar shows the current position rating of the active engine.
The way it works depends on the engine used. Classical chess programs usually evaluate by pawn units. If the white bar is one box above the center marker, white is one pawn unit better.
Neural engines, so-called NNUE engines, evaluate according to the probability of winning.
Another special feature can be very weak game strengths, such as the Play&Win profile in Komodo 14.1. This engine shifts the internal position evaluation in such a way that it evaluates itself significantly stronger than it actually stands, in order to sacrifice material more willingly.
So the display of the rating bar will work completely different depending on the engine and the profile or settings of the engine.