Dr. Ebraheim’s educational animated video describes what is a ganglion cyst.

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A ganglion cyst is a benign cyst that is filled with a jelly-like fluid. Ganglion means a “knot”. It is not a malignant tumor; it is just a soft tissue mass. Ganglion cysts are not cancerous and will not spread. This mass may change in size, and it may grow slowly. Ganglion cysts usually do not cause any harm and if the patient finds it, the patient is usually more concerned about how the cyst looks. These cysts can put pressure on the nerves, vessels, and tendons which can cause problems for the patient. Ganglion cysts can occur anywhere. They usually occur at the wrist area, however ganglion cysts may occur at the foot (usually at the top of the foot). Ganglion cysts can occur around the shoulder, around the knee, or even occur in the bone itself. Ganglion cysts around the foot or the wrist will usually transilluminate because it is not a solid cyst; there is fluid inside the ganglion cyst. Ganglion cysts will transilluminate.
Treatment of Ganglion Cysts
Ganglion cysts that do not bother the patient are usually left alone. Aspiration will decompress the ganglia and decrease the pressure on the adjacent structures. Surgery is done as a last resort to treat ganglion cysts. It is usually done when the ganglion cyst causes the patient pain or harm such as pressure being placed on the nerves, vessels, and tendons.