A pimple. It sometimes happens. (My husband got one once, and his doctor gave him an ointment to put on it every night and said it wasn't a big deal.) If you are worried, see your doctor.
Well, pimples come in a lot of colors. White means you had a blocked pore and now there are dead skin cells inside and probably some bacteria. I don't suggest you pop it, because of the bacteria. But if you were to put a warm compress on it (a very clean, warm, wet cloth) or to sit and soak in a warm bath, it might open and drain. Also, you can use something like Neosporin or Polysporin on it, no matter if you soak it or not. (Though don't touch it with your fingers if you haven't scrubbed your hands, your fingers have a lot of bacteria on them and you would risk making things worse.) I don't remember what color the one my husband had, just remember that the doctor was very unconcerned.