Hard lump on the side of my left knee about the size of a penny. What could it be?

Simple vs serious. Our bodies always have things that grow and change. Skin, fat, muscle, lymph nodes, bones, can change shape, get larger, more prominent due to many factors. The vast majority are not cancer but simply unusual growths like fatty tumors, cysts, skin tags, moles, etc. Most cause no symptoms and can be watched. If they cause symptoms or change colors, bleed, bruise or hurt, then see your doctor.
Several things. Your doctor needs to feel and palpate the lump to be able to tell you what it is.