I have a hard and tender abcess on the back of my tongue what is it?

Enlarged taste buds. Though most sore tongues are nothing to worry about, they are usually cause by some type of irritation. You should consult a doctor if you have a lump or sore on your tongue that doesn't go away within a week or two.
Tongue lesion. This type of a lesion is very concerning and I suggest that you visit an oral maxillofacial surgeon as soon as possible for a definitive diagnosis.
Hard to say. Please see your local dentist. It could be anything from an enlarged taste bud, to an infection, to oral cancer. A clinical exam is necessary to diagnose your specific condition. Please seek help soon, and take care...