The lineup screen is where you will manage which of your players will be active or benched for each game opportunity.
The column on the far right shows you the total game opportunities allowed for each position and how many players you have selected to play. The example in the screen below shows that you can have a maximum of 18 games played with 3 Outfielders.
The columns in the middle are game opportunities by date, time, and who they play against. You can also see your projected points and toggle the player on or off.
You will need to decide which players' game opportunities are used to calculate the points earned while staying within the allowed total amount of games for each position.
To select which players are used for each game opportunities use the toggles. Toggle to the right (grey) means you have chosen the player. Toggle to the left (red) means the player is not selected.
Under each player position, to the left of the player headshot, you will see the 
. This is the player 'Swap' button. To know more about how to swap players see How do I add/drop (swap) players from my lineup?