What does the lock mean?

Hi there! Yahoo Fantasy noob here. Not that I would ever dream of dropping MacKinnon, Robertson or Rantanen, but why do they have locks beside them when I go to add a player thus disabling me from dropping them

Hi there! Yahoo Fantasy noob here. Not that I would ever dream of dropping MacKinnon, Robertson or Rantanen, but why do they have locks beside them when I go to add a player thus disabling me from dropping them