Backbonding in molecules like BF3
$\ce{BF3}$ has trigonal plannar structure all the three $\ce{B-F}$ bonds lie in plane and thus p-orbitals of boron and fluorine become parallel.

Boron has empty p-orbital and p-orbital of fluorine contains lone pair, and hence  boron acts as Lewis acid and fluorine as Lewis base. Fluorine donates its lone pair to boron and this bonding is called backbonding.