Basically it hides certain attributes/stats (eg pace tackling etc) of a player if he is more unknown to the world. For example if you were to click on a 17 year old from a team in the Russian league, alot of his stats would be hidden due to the realism of not knowing how good a football he really is.
Yes as Danny said basically it hides some of the attributes of players/player from regions your club does not have good knowledge of to add realism. You can reveal their attributes by scouting the player.