It means the player's current ability can potentially go up to
199. 
You need
to have good coaches and training schedules to allow him to fully
develop so well. Right now in my game, I got a striker whose
potential is 199. He's 20 years old now and have 145 ability value.
I want to see whether he can go all the way to 199.
Also, a player's ability can increase dramatically until the age of
24. After 24, ability can still go up, but slower. At the age of
32, most players' ability stat will go down.
There are some players, who after 32 can retain their current
ability value. For example, in my game, although Iker Casillias is
nearly 40, his current ability stat still hovers around 170+.
Another important point to note that there are also two important
stats, namely Consistency and Important Matches, these two
determine how consistent can the player perform. The highest is 20
for both. So you may find cases where a player with 165 ability but
20 for consistency will do better than someone with say, 180
ability but 5 consistency. For the
info, around 165 is considered "world class" in the game.
Lastly, like to mention that Consistency and Important Matches
stats are hidden as well. Also, I think every player can
potentially go to 20/20 though it's not reflected in the genie
scout. I suppose good trainings and tutelages under good senior
players will help to push these two stats up.
