Resonance and hyper conjugation
Resonance is a way of representing the delocalized electrons within a molecule with different structures when a single Lewis formula is not enough to express the bonding of electrons.
There are various structures that can be made from the actual molecular structure; the average structure between canonical forms that explains the experimental observations are called as resonance hybrid. Point that should be noted is that these structures vary only in respect of the position of the electrons not in the position of nuclei.
There is an extension in the molecular orbital when there is an interaction between sigma bond and empty/partially filled π-orbitals. This results in hyperconjugation which increases the stability. Bond length, dipole moments and heat of formation of molecules is affected due to hyperconjugation.