Just an extra growth of skin that looks like a tag, hence the name. They usually just freeze them off with liquid nitrogen at the doc’s office. No big deal, usually. Anything unusual on the skin should be checked by a doctor, though. All though it is probably not anything big (such as skin cancer), it is nothing to fool around without, without a professional opinion, as the serious stuff can sometimes get out of hand quickly, when it could have been easy to take care of, if one had acted fast.
When visiting the dermatologist, ask what signs to look for in the future and have him/her check out anything else on your skin. It’s also a good time to ask about recommendations for sun screen, etc. for your particular skin type.