If you are a cat owner, you are undoubtedly very familiar with the backside of your cat. Mainly because they insist on shoving it in your face for some reason.
Cats use body language to communicate. A butt in the face is, oddly enough, a good thing! It can be a greeting, a sign of approval, or a way to show they are not threatened.
When your cat lifts its tail and exposes those glands to you, it is a way for them to say they are marking you. They do the same thing by rubbing up against you with their faces or putting their paws on your face.