They are called performance ratings. These are meaningful ratings for you to gauge how they fare against other players, and is a good measuring stick not only to know how good and effective he was in that game (different matchups can yield different numbers) but also a good indication of his form compared to his gameshape (you perform better when you have good game shape obviously). Usually players get the highest ratings when they play in their best positions, but sometimes people play their players out of position to take advantage of certain matchups. Ratings however don't tell the whole story; for example some people play PG at C in isolation/mismatch tactics and get low ratings but the PG torches the C with his shooting (you got to remember adjust your defensive assignments or the C will own the PG on the other end :P)