What is an alkyl halide?
Let's just take it word for word.
The alkyl group here is a propyl group; a three-carbon alkyl group. The halide portion is evidently the bromide substituent. Just as bromine is a halogen, it as a substituent is a halide.
Halogens tend to be fairly electronegative. Or, at least enough that we consider alkyl halides to be reactive at the carbon directly connected to the halide. The electron-withdrawing quality of the halide polarizes the bond towards the halide, meaning most of the electron density is found nearer the halide than near the directly-attached carbon. Evidently, that means the halide possesses a partial negative charge.
So, the aforementioned directly-attached carbon has a partial positive charge, and we call that an electrophilic center.
Because of that, it is reactive towards electron donation from a nucleophile, simply because electrons are attracted to a partial positive charge.