Tendons run in sheaths, similar to the cables on a bicycle.
Sometimes a minor trauma can cause the sheath to become inflamed and a small cyst can pop up. This is called a ganglion cyst
. They are usually painless and sometimes go away on their own. If it isn't bothering you then you likely don't need to do anything with it. If it is bothering you, see your doc about removing it.
Answer assisted by David Miller, Medical Student