This is a type of currency in the game. They are purple orbs and are collected by moving over them. There are a lot in the game. You should have no trouble collecting enough to get all of the related achievements. By chapter 6 it's possible to already have 1000 of them. If you replay chapters it will all count towards your total. You can replay the start of a chapter that has a lot of them, then quit out once you hit a checkpoint. But you should be able to just grab them all while playing the game naturally.
Note: These are often used as a hint of what direction to go in the game. They do not appear on hard difficulty.