Hi Alicka. I would best describe "niche" fragrances as those that are not produced by a designer brand, but instead come from a house that produces nothing *but* fragrances. Typically they are only found in boutiques, and not at your average mall store.
Examples: CDG, L'Artisan, LV, SMN, Creed. Of course there are certain houses that may fall into a gray area, in which some may consider them niche, and some don't For example, Caron and Guerlain.
Personally, I prefer to stay away from such categories, and trust my nose to tell me if something smells good or not, regardless of where it came from.
Hope that helped answered your question!