Ree score is the very heart of rankade.
It's the score that represents the skill of each player within a group and determines the ranking. The higher a player's ree score, the 'stronger' that player is.
Ree is only relative, i.e. it means something only among the members of a group. It makes no sense at all to compare your ree within a group with your or someone else's ree scores in other groups.
Here you can find some more info about ree algorithm, and a comparison between most known ranking systems.