Player Cards are your players! Collect them from Card Packs to increase your player base!
From the Dugout, you can examine Player Cards you own, and ones you haven't unlocked yet. You can tap on them to view their Salary (how much of your Salary Cap they use when on your Roster), see their Abilities, and Upgrade them once you've collected enough Duplicates!
Level up your Player Cards to make them more effective!