Quick answer: Your cat is returning the grooming — when you pet her, she responds by licking you as she would another trusted cat.
Cats in social groups groom each other as a bonding activity. When you run your hand along your cat, she interprets it as grooming and instinctively reciprocates. It's a two-way exchange of affection.
If you've reacted warmly to being licked before, your cat has learned that licking gets a positive response. She may continue and even intensify the behavior because it works.
Yes — it's harmless for most people. Cat saliva isn't dangerous for healthy adults in normal quantities. If you have sensitive skin or cat allergies, you may notice mild irritation where she licked.
This is one of the clearest signs your cat sees you as a trusted companion. Enjoy it.