It means that it is now chewy and hard to bite through or tear apart with your teeth. Tough meat will be real chewy and difficult to eat where a piece of Tenderloin will be almost like butter sometimes. You can break it apart by just pulling it with a fork as opposed to cutting with a knife.
If on the other hand you are speaking of the act of tenderness, it is being gentle and understanding with someone that is going through a difficult time or being soft and loving.