It's a complicated computer algorithm that's been designed and redesigned and tested over millions of positions for accuracy.
It's just a rough evaluation on how far one is ahead - +0.28 means that white is roughly 3/10 of a "pawn" in advantage... you shouldn't interpret it this way, but instead you should use your brain and examine the chess board to see what's going on, and just in account that white might have a comfortable position / slight edge / positional is dynamically equal, etc.
when you analyse a game and there is a dynamic indication of something. is this a measure of how far a player is ahead in terms of probable outcome? if so, is this measured in a similar way to the crude method of counting points (5 for a rook etc)? and if so, what is taken into account?