Your team consists of 10 starting players plus reserves (number determined when league is created). This means that each week you can adjust your lineup by changing the starting players. Only players that are starting that week will earn you points.
Players become ‘locked’ either in or out of your starting lineup when their game for that week kicks off. So you have until kickoff to swap out starting players. Of course, if the player you want to swap in to your starting lineup has already started their game, you cannot then bring them into your starting lineup as they are locked out.