That means that your player fulfils some requirements set by the U21 coach of the player's country.
You may contact the U21 coach and ask if they're really interested in the player or not. If they are, they may give you some tips regarding how to train that player in order to give him chances to play for his country's U21.
I can't look at the links in your post because that page is blocked on my PC (at work) but if I can have a look later today if nobody has answered by then.
We have both said a lot of things that you are going to regret.