The chemical formula for iron(II) bromide is FeBr2. "Fe" represents iron; "Br" represents bromium. When numbers are included in formulas, they indicate how many atoms are present in the molecule. "2" indicates two atoms of bromine. If no number is present, then only 1 atom is present in the molecule.
The Roman numeral II in the parenthesis after iron indicates the iron has a charge of plus two. Each bromine atom carries a charge of minus one, requiring the molecule to contain two atoms to balance the charges. The chemical formula for iron(III) bromide is written as FeBr3 and contains a third bromine atom.