Skin lump, that is painless, soft on touch and usually movable is probably a benign tumor of soft tissue, known as lipoma (composed of fatty tissue).
Lipomas are usually relatively small (diameter of 1-3 cm), located just below the surface of the skin at the usual sites like trunk, thighs and the forearms.
They are harmless in most of the cases and do not need treatment.
They must be examined if they start to change their size (getting bigger) and if they become painful.
Best wishes!