Are you familiar with the term "grignard reagent"? The molecule on the left side of the arrow is one (simplified with: RMgX (R=carbonchain, X=halogene)).
The Br molecule which is attached to toluene (simplified as RX) reacts as a leaving group when RMgX attacks it.
I think chenbeier made it all clear to you yesterday. But maybe this makes it more clear.
Also a tip: If you get a question where bromocyclohexane reacts with Mg (as the reagent) it will also form the molecule on the left side of the arrow.