I have become a fallen ranger. What does that mean and should I care?

It means that your reputation got to low to be the class Ranger
Yes it is a bad thing if your reputetion gets to low shopkeepers will raise their prices and eventullay not sell.

Beyond the rule book, to fall means that your ranger (or paladin), who must be good by deed and by nature, has behaved too dastardly for the gods to tolerate, and therefore your skills and abilities are taken from you.

If you enjoy robbing people, looting, killing innocents, you should choose a character of chaotic neutral (= insane) or any evil alignment. Can't be a ranger or a paladin, though.