A roster in fantasy football is a group of players that make up a team. Fantasy football rosters have players at multiple positions, with varying degrees of skill. Solid rosters tend to have a number of talented players that perform well on a consistent basis, giving owners a realistic shot at winning the league.
Each team in standard fantasy football leagues is allowed up to 16 players, with 9 starters being used each week and 7 players being left on the bench. Therefore, only the point outputs of the 9 players a manager decides to 'start' are counted towards the team's total score for that week.
If a team owner is already capped at 16 players, a player currently on the roster must be dropped in order to add a free agent. The same logic applies to trading with other teams, as 'two for one' deals in which one manager receives two players in exchange for one must drop a player currently on the roster to make room for the extra acquisition.
When assembling a roster on draft day, it is important for fantasy owners to fill the most important positions earlier in the draft and wait until the later rounds to fill less important positions. Some positions are deemed more important than others based on the potential point output they bring to a team. For example, because a running back is more likely to contribute a large point total than a kicker due to a higher usage rate, fantasy owners will often draft running backs early and kickers later on.