to add depth to the secondary =?

Secondary is a reference to the defensive team's backfield--those players who are farther back from the line where the two teams face one another, and whose purpose is to defend against the pass, to defend against the ball being thrown down field. Formally speaking, they are known as the defensive secondary.

No, it means to add skill. Defensive linemen on a National Football League team are commonly over six feet tall, and commonly weigh well over 250 pounds. Even in the defensive secondary, the safeties and the defensive backs are large, heavy men in comparison to the general population, although they will certainly be smaller than the defensive linemen. A man who is five foot seven inches tall and weighs 160 pounds can hardly be said to add weight to such a group. I don't know the player referred to, but i suspect, because of the comment, that he is fast and agile, and skillful at defending against the forward pass (the other team throwing the ball), and so his skill "adds depth" in the sense of making the defensive secondary more effective.