what does the Sun's dynamo do?

We don't fully understand how this works. But we know that the solar dynamo causes the change in the number of sunspots. The Sun rotates faster at the equator than at the poles. The rotation of the Sun twists and tangles up the solar magnetic field inside the Sun until eventually it gets itself into a right mess.

The magnetic field starts to break through the solar surface and produce lots of active regions and flares. The solar cycle, which is described in the sunspot section by Keith and Andrew, is produced.

Do other stars have their own dynamo?
Yes, many of them do. All that is needed is rotation, a magnetic field and a hot ionised gas. Other stars have cycles of activity, from very quiet to very active, just like our own star, the Sun.