What do the arrows next to the skills do?

[Jarold]

I know that up arrow means you gain, and that the lock means can't gain or something. But what does the down arrow do? I was hoping it meant that the skill will decrease. Can anyone tell me.

[Atraxi]

if the skill is pointed down, it will decrease if you gain in another skill and you are at your skill cap (700).

If you haven't yet reached the total skill cap, the skill won't decrease.