What is Hybridisation?

Carbon in methane has the configuration $2s^2 2p^2$, but it forms four equal bonds. One $s$ and three $p$ orbitals mix to make four $sp^3$ orbitals pointing to the corners of a tetrahedron, at $109.5^\circ$. Mixing one $s$ and two $p$ gives three $sp^2$ orbitals in a plane at $120^\circ$, and one $s$ with one $p$ gives two $sp$ orbitals in a line at $180^\circ$.