Hybridisation is intermiximg of atomic orbitals of approximately same enargy to give same number of molecular orbitals of same energy and same shape.
It is an hypothetical concept proposed to explain the shapes of various molecules.
For example in methane CH4 atomic orbitals of carbon atomm undergo hybridisation as discussed below:
The ground state electronic configuration of carbon is 1s2 2s2 2px1 2py12pz0
excited state configuration is 1s2 2s1 2px1 2py1 2pz1
In the excited coniguration 1 electron from 2s2 is promoted to empty 2pz orbital.
Now one 2s and all the three 2p orbitals undrgo hybridisation to form 4 molecular orbitals which have same energy and shape. these 4 hybridised orbitals then combine to with 4 1s orbitals of 4 hydrogen atoms. Since hybridisation of central atom whichis carbon in methane its shape will be tetrahedral.