Major League Soccer’s secondary transfer window is set to close on August 8th, after being open since July 10th. While a team can add a player AFTER the August 8th date, that player is unable to be played in any official MLS matches until the next transfer window (beginning in February).
In addition to transferring players in, any intra-league trades must occur during the window. However, teams can swap draft picks, allocation money, etc. outside of the two transfer windows. Teams can also get rid of players outside of the transfer windows; basically, it just means new players cannot be added to the club.