A trace of boron, which carries only three valence electrons, is added to the silicon crystal. Four bonds need filling but one electron is missing, so a vacancy is left in a bond. That vacancy is a hole.
When an electron hops in from a neighbouring bond, the vacancy moves to where that electron came from. Repeat this and the vacancy travels in the opposite direction to the electrons. The electrons are what move, but the vacancy is what is seen moving.
A hole can therefore be treated as a particle carrying positive charge. The boron that took the electron in stays fixed in the lattice as a negative ion. Because the charge being carried is positive, this material is called p-type.