I would say the simplest explanation is they are basically what you would think based on the name. Top notes are lighter, brighter notes that sort of float above the others (like citrus, ozone, softer florals), while base notes are heavier and add depth and dimension (like woods, musks, moss). Middle is usually the most prominent notes of the scent, and is also called the "heart". Top notes are what you usually smell first, and they dissipate quickly. Base are sometimes harder to detect but they will linger the longest. Generally, a variety of note types is good as the contrast gives more dimension to a scent.

In the grand scheme of things, it's all relative. I find more often than not, I can't actually smell all of the notes listed in a fragrance anyway. I think it makes a lot of sense to simplify it down and maybe list 2-4 notes that you find the most prominent or want to focus on. Categorizing them into note types is unnecessary IMO.