What is hybridization?

Example: A Mg atom has only 2 electrons in its valence shell. In the ground state, they are paired in the 3s orbital, so this state cannot bond to other atoms. However, the 3s orbital can combine with one of the 3p orbitals to form two new $s p$ orbitals. When one electron is in each orbital, then it is possible to form two equivalent bonds to 2 hydrogen atoms to form $M g {H}_{2}$.
This is also the way that carbon can form 4 equivalent bonds. The 2s and 2p orbitals combine to form 4 equivalent $s {p}^{3}$ hybridized orbitals. When bonded to 4 H atoms, you get methane.