Here's how to understand it:

* Boron (B): Boron is a metalloid with a +3 charge in its most common ionic state.

* Fluorine (F): Fluorine is a halogen with a -1 charge in its most common ionic state.

To form a neutral compound, one boron atom needs to bond with three fluorine atoms (3 x -1 = -3 charge, which balances the +3 charge of boron).