As your wort comes to the boil, it is normal for foam to appear. 

You should use your paddle to pat it down otherwise the foam will start to rise and form a smooth surface. 

If the foam suddenly overflows, this is a boil-over. If it looks like it is going to boil over, either lower the heat or spray the surface with water from a spray bottle.

The foam is caused by proteins in the wort coagulating due to the rolling action of the boil. To try and prevent this you should make sure to stir when bringing the wort to the boil and occasionally while boiling.