Stumbling is falling down while one is trying to walk. Disobeying one point of the law is spoken of as if it were stumbling while walking. (See: [[rc://en/ta/man/translate/figs-metaphor]])
Here "you" means "each one of you." Although James was writing to many Jewish believers, in this case, he used the singular form as if he was writing to each person individually. (See: [[rc://en/ta/man/translate/figs-you]])