Hybridization is a term coined by a scientist Pauling in 1931.

It is an imaginary concept which shows the intermixing of the orbitals to form a new one, which is of approachable in energy, to make a bond.

It will be more easy to understand with an example, so let say we have to form a bond between the C atom and a H atom, we know that the bond can be formed by the overlapping of the orbitals, so for the proper overlapping of the orbitals, the energy of the orbital must be comparable, but in this case it is not, hence, the orbitals of C, i.e s and p orbitals of C get mixed and form a new sp³ orbital, which can easily overlap with the s orbital od the H-atom. Hence, the bond can be formed easily now after hybridisation.

Now, checking the hybridisation of the carbon atoms in the molecule CH₂ = C = CH₂.

The molecule CH₂ = C = CH₂ contains both sp as well as the sp² hybridised carbon atoms.

The two terminal Carbon atoms are forming three sigma bonds so, the hybridisation will be sp² , whereas the central carbon atom is forming only two sigma bonds so, the hybridisation will be sp.

As we know that the hybridisation of any atoms is predicted by considering the number of the sigma bond formed by that particular atom and the loan pair of electrons present at the atom.

The molecule will be planar as the sp² hybridisation leads to trigonal planar shape and sp leads to formation of linear geometry.

Hence, the molecule CH₂ = C = CH₂ is overall planar molecule.