Hybridisation refers to the mixing atomic orbitals to valence shell of an atom to form new hybrid orbitals more suitable for the pairing of
electrons to form chemical bonds.
For example, in methane ,in which a carbon atom forms four single bonds, the valence-shell 2s orbital combines with three valence-shell 2p orbitals to form
four equivalent sp3 mixed orbitals in a tetrahedral arrangement around the carbon to horm bond with four different H atoms.See the animation: formation of Methane.
Hybrid orbitals are useful in the explanation of molecular geometry and atomic bonding properties. Usually hybrid orbitals are formed by mixing atomic orbitals of comparable energies.