A silicon atom carries four valence electrons, and it shares every one of them with a neighbour, one electron each way. Four bonds reach out in four directions, the atoms settle into a regular pattern, and the result is a crystal. In that state every electron is held in a bond, so nothing is free to move.
As the temperature rises, thermal jitter can knock a single electron out of a bond. That electron is then free to wander through the crystal, and the bond it left behind holds a vacancy. The vacancy is a hole: electrons hop in from neighbouring bonds, so it behaves like a particle carrying positive charge.
Free electrons and holes are always born in pairs. After a while the electron falls back into the hole and the pair disappears. That is all that happens in pure silicon, and it leaves very little charge to carry. Semiconductors become useful only once an impurity is added to raise either the electrons or the holes.