Salary Cap is a value that tells you which cards you can put on your Roster.

On the top left of each Player Card, you'll see their Salary. This is how much it costs to field in your Roster. If the total of each Player Card's Salary is under your Salary Cap, you can play with the selected team. Otherwise, you'll need to put players with a lower Salary on your team until you can raise your Team Level to gain a higher Salary Cap.