Don't know what U stands for, but 2U rackets are heavier than 3U rackets. Its a scale from U-4U not sure if there is 5U. But anyway, U is the heaviest about 104g, 2U is about 94g, 3U is about 89g, 4U is about 81g. I've also heard of 2L and stuff, but not sure how heavy that is.....
These are a measure of the weight of a racket: 3L: 100 - 104 grams U: 95 - 99 grams 2U: 90 - 94 grams 3U: 85 - 89 grams 4U: 80 - 84 grams ?