As you continue to build your team, you will start collecting players who will become inactive due to limited space in your active roster.

The Roster Decision setting determines when new players who are better than your current lineup will take the place of old players, moving them to the Inactives roster. (Team tab> Gameplay tab> Strategy). "Current" will add the highest current overall, and Roster Decision - "Potential" will trade in the lowest max overall. 

You can trade your inactive players for the resources that you can use to improve the players you do want on your roster by going to Team> Line Up > Trade-In and then pressing the "Trade-in All Inactive" button.

Once your Inactive roster is full, your inactive player will trade automatically. 

Note that you may "lock" players (from the Batters or Pitchers tab) at the top of your Team screen in your roster if you do not want them substituted or traded.