When a H–C bond is attached to an unsaturated system such as double bond or a benzene ring, the sigma (σ) electrons of the H–C bond interact or enter into conjugation with the unsaturated system.
The interactions between the electrons of systems (multiple bonds) and the adjacent bonds (single bonds) of the substituent groups in organic compounds is called hyperconjugation.
The concept of hyperconjugation was developed by Baker and Nathan and is also known as Baker and Nathan effect.
For example in propene
refer to my image for understanding this effect in propene