What do you mean by conjugation? It is the overlap of p-orbitals across a sigma bond. As we know the hybridization is sp2 there is an unhybridized p orbital per carbon atom. When a compound or molecule has alternating single bonds and pi bonds, the unhybridized p orbitals can get overlap with each other, forming an electron cloud. The electron in those p orbitals then gets delocalized inside this electron cloud. This phenomenon of this overlapping of p orbitals is known as conjugation.