Plantar fibroma. Ganglion cyst and many other possibilities. I recommend you get evaluated. The history is important as well. You may need diagnostic testing such as x-ray, sonogram, or MRI if the diagnosis is not obvious. Do not wait to get this taken care of because a "lump" is usually not normal.
Ganglion or Fibroma. A ganglion (fluid filled sac), fiborma (dense connective tissue).