Whats the MLS cup lads, is that like the playoffs/league?
MLS Cup = winner of the post-season knockout tournement, contested amongst the 10 teams who finish the league with the best record.
Supporters Shield = team who finishes the regular season with the best record.
In American sports culture, the MLS Cup champion is the real champion. For instance, in baseball/NFL/NHL/NBA, no one cares who finished the regular season with the best record. The champion is whoever wins the post season tourney, culminating with the World Series/Super Bowl, Stanley Cup Finals, NBA Finals.
Supporters Shield means something (to the extent anything means anything in MLS) only because MLS fans know the traditions of World Football and are trying to copy it. Supporters Shield also gets an auto bid to CONCACAF Champions League.
We also have US Open Cup, which is exactly like the FA Cup, only a billion times less important.