Your natural coloring — skin, hair, and eyes — has its own warmth, depth, and intensity. Clothing and makeup colors that echo those same qualities make your features look brighter and more alive; colors that clash with them can make you look tired, sallow, or washed out even if the color is "pretty" on its own.
"Color seasons" (Spring, Summer, Autumn, Winter, each with subtypes) are a shorthand for groups of colors that share the same warmth/coolness, depth, and clarity as different coloring types — so once you know your season, you have a ready-made palette that's likely to flatter you.