B has an electronic configuration of 1s22s22p1 and undergoes sp2hybridisation and then bonds with 3 fluorine atoms to yield BF3.
B (ground state) 2s :

↿⇂

2p :

↿

B (excited state) 2s :

↿⇂

2p :

↿⇂

↿⇂

[sp2 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)