A deadline is the latest time or date by which your lineup must be set up for the upcoming gameweek.
The gameweek deadline is usually the start of the first match. After the deadline no changes can be made. In case you forgot to edit your lineup, or of one of your players did not play, the auto subs can prevent or reduce a loss of points.
A trade deadline is the last day of the season in which trades can be processed. In most NFL Fantasy games this occurs a couple of weeks before the end of the regular season. It’s a preventative measure so eliminated teams can’t sandbag their team by trading all of their best players to the same team.