Combo is a kind of skill that would allow you to make more shots before the "final blow" (this pushes away the opponents). If you haven't read any books combo, you can make 3 attacks before the "final blow". When you have read 1 combo book, you can make 4 attacks before the "final blow". When you have read 2 combo books, you can make 5 attacks before the "final blow". Combo is a Skill, which can be used with a right click. At level 30 you can read combo to 1, this is either done by using the "The Art Of Combo" book or the "Combo Mastery" book. From level 50 you can read Combo to 2, this is done by reading the "Record Of Combo" book.
Combo is useful in PVM, because you can make more attacks while fighting on the ground without making the mobs go away.