B has an electronic configuration of $1s^2 2s^2 2p^1$ and undergoes $sp^2$hybridisation and then bonds with 3 fluorine atoms to yield $BF_3$. B (ground state) 2s :

↿⇂

2p :

↿

B (excited state) 2s :

↿⇂

2p :

↿⇂

↿⇂

[$sp^2$ hybrid orbitals] But, inspite of this bonding it remains electron deficient i.e., it does not have 8 electrons around it in the outermost shell; it has only 6 electrons in the outermost shell. This electron deficiency coupled with the fact that it has an empty 2p orbital which can accept more electrons making it a Lewis acid. (Lewis acids are compounds that can accept electrons)