Some molecules do not follow the octet rule that states that elements gain or lose electrons to become stable. BF3 is an example of this.
Boron in BF3 is electron deficient as it does not have a complete shell of outer electrons.
BF3 is susceptible to bonding with molecules that have a lone pair of electrons in their outer shell for example NH3. They form a coordinate covalent bond where the lone pair of electrons in being shared not transferred.
In a coordinate bond, both bonding electrons come from the same atom.