Hyper conjugation

Both carbons of an alkene are sp² hybridized. Though, this is altered on creation of the carbocation as shown in figure. While an alkene reacts with an electrophile like a proton, both electrons in the π bond are employed to create a new σ bond to the electrophile.

Figure:  Hybridization of alkene and carbocation.

As a result, the carbon that gains the electrophile becomes a sp³ center. Another carbon consisting of the positive charge remains like a sp² center. The meaning of this is that it has three sp² hybridized orbitals (employed for the three σ bonds still present) and one vacant 2p orbital that is not take part in bonding. Hyper conjugation includes the overlap of the vacant 2p orbital with a neighboring C-H σ-bond orbital.

Figure:  Hyper conjugation.