The 'U' stands for Under. Most leagues in the United States recognize August 1st as the universal age cutoff date for the upcoming Fall AND Spring seasons. So if a child is 'U'nder 8 years old on July 31st, but wasn't 'U'nder 7, they would be in the U8 division. Note a child born on August 1st turning 8 is considered to still be Under 8 for the soccer year. Some leagues have age divisions that span multiple years, so a league might only have U6, U8, and U10 divisions while others might have U5, U6, U7, U8, etc.