The basic idea is that you have high or low contrast between skin, eyes, and hair and predominately cool or warm undertones. Thus, you supposedly look best in colors with an undertone and intensity that complement these parameters. For example, people with low contrast may look washed out in or overpowered by bold colors that would work with high contrast.
I've been looking at a bunch of different season selectors online, and I think mine is Chewbacca because I do not make sense. My skin is pretty damn white (Revlon's palest shade of foundation is a near perfect match) and its undertones are definitely blue. My hair is blonde but with enough strawberry that more than one person has referred to me as a redhead. My blue-gray eyes are pretty much irrelevant to this conflict between "cool" and "warm". The closest I can squish myself into is "summer"...but for years, I've preferred deep clothing colors supposedly reserved for winters. I always thought my hair looked muddy against lighter colors, whereas black and dark jewel tones make my hair shine and my skin glow. (Black light has made the underside of my arm glow, but that's another story.) And on top of all of that, jet black hair does look natural on me once I cover my freckles and darken my brows. When in costume, I was asked whether that was all my real hair.
I think that when your skin is pale enough, it will work with a quite wide variety of hair color (and I wouldn't be surprised if very dark skin were similar), and my natural hair color is strong enough that it doesn't get overpowered by bold colors. Though I love bold colors, I've bought clothing with more muted tones that I thought looked good on me (like a medium grayish blue), and I've just learned that those are summer colors. So maybe I'm a Christmas in July.