it is a type of bonding which take place between atoms in a compound ;in which one atom has lone pair of electron and other has vacant orbital placed adjacent to each other.
A compound which posses back bonding has pi bonding character because it occurs after formation of sigma bond.
Usually ; back bonding causes molecule to get stable as it completes octet.
for ex. in borontriflouride ,
back bonding causes decrease in bond length and increase in bond order.