What is orbital hybridisation and its relevance?

In atoms, electrons arrange in different shells (K, L, M...), each holding a number of orbitals (s, p, d, f...). Each orbital holds a maximum of 2 electrons. In some circumstances, a number of orbitals mix to form new, hybrid orbitals (hence the name 'hybridisation'). The number of orbitals that enter hybridisation is equal to the number of resulting orbitals. The hybrid orbitals' name is created by stating the letter and the number of initial orbitals used (e.g. sp3 means one 's' orbital mixed with three 'p' orbitals, resulting in four sp3 orbitals). Hybrid orbitals have different shapes, energy levels and properties than the forming orbitals.

The relevance of hybridisation lies in the circumstances in which this phenomenon happens. For example, a Carbon atom is able to create single, double and triple bonds because of the ways in which its orbitals can hybridise. For a Carbon atom to create a double bond, one 's' orbital combines with two 'p' orbitals. This results in three sp2 orbitals and one unhybridised 'p' orbital. In the double bond, one sp2 orbital and the unhybridised 'p' orbital are both connected to another atom. Drawings are provided for the shapes of the initial and hybridised orbitals.