Welcome to the game!
5x5 and 6x6 are both types of roto, or rotisserie (after the eatery where the rules were devised), scoring, in which teams compete in separate categories (5 hitting, 5 pitching in 5x5, usually R, HR, RBI, SB, AVG; W, SV, K, ERA, WHIP - 6 each in 6x6). The worst team in each category gets one point, the second-worst two points, and so on.
For example, if there are ten teams in your league, and you're seventh in total HR, you'll receive four points. If you win the saves category, you gain ten points. The points for all the categories are added up, and the team with the most points wins.
Clear as mud?