Just like our earth, our sun's internal structure has different zones. Underneath the solar surface, which is called the Photospehere, is a 'Convection Zone'. This is where the internal material comes up, cools down and then falls back. Hence we see a boiling effect on the surface.
Here is the super magnified version of the same image.. every bright round dot is called a granule which is about 700-1000 km in size, having a life of about 10 minutes.