alternate abilities or AAs are ways to upgrade your character in certian areas. you have 3 different lvls of AAs that you can work with. the first lvl starts at lvl 51, you can upgrade anything from dex to how fast you run you need 6 points in this area before you can go to the next upgrade. when you hit 55 you get the second set called archtype these deal more with the general areas of your character. AAs in this area for rangers deal mostly with ATK. finally the third lvl opens up at lvl 59 these are class specific. for example, I am a lvl 62 ranger on stormhammer i have 3 points in innate run speed (makes me run faster than a normal person), and 3 points in regen( i regen HPs faster), in my archtype i have combat fury 3( each lvl of this AA is double the points 2,4,6) combat fury 3 makes it so i can crit with my swords and hit harder. in my class specific i have Endless Quiver( 9 AA points, and this is a must for all rangers) Endless quiver is never running out of arrows again. i also have Archery Mastery 1 (3 aa points) this is improved damage with a bow. when you see the rangers hitting for 2k a hit this is the format that they are using. hope this helps a bit and good luck on deciding were to put your AA points.
oh btw it is said that each AA point is equal to doing lvl 51 all over again.