You can "Like" a player to keep track of the players you like. Players that you "Like" are prioritized by the optimizer higher than non-liked players, so they will be included in the generated lineups. This feature lets you "Like" multiple players at a specific position instead of locking just one player at that position.
In addition, the "Like" count indicates how many times the community has liked any given player. This is a useful way to sanity check your picks or fade the competition.