Yes, it will reduce it a little. A shotgun has an area that it picks up ideally, and the other areas not so much. It's ideal for speech in an environment where you want to isolate it a little more to not capture someone else's speech or just noise in general near by. When you record, you record it at a higher gain so that you can drop it in post-processing and by dropping it in post, the ambient virtually goes away, and all that is left was the speech that was in the area the shotgun mic picked up mostly. I use this kind of microphone for kids and family stuff where you're recording someone talking, singing, etc and you want them isolated.
You don't want a shotgun mic if recording multiple people spread out, or a concert, or if you want ambient sound to be recorded naturally. Use an omni or stereo mic for that.