HSL describes any colour with three answers. Hue is where it sits on the colour wheel, measured as an angle: red at one point, yellow a sixth of the way round, green a third, and so on. Saturation is how pure it is, from grey to fully vivid. Luminance is how light it is. Changing only the hue of a band keeps the shading, highlights and texture of the original, which is why a shirt recoloured this way still shows its folds.
Each of the eight bands covers a slice of the wheel rather than one exact shade. The band called Red also reaches the pinkish and slightly orange reds next to it, and Aqua sits between greens and blues, catching turquoise water and teal paint.
Why some swaps look real and others don’t
Colours of equal saturation are not equally light to the eye. A pure yellow looks far brighter than a pure blue, and a vivid green looks lighter than a vivid red. So when you swing a hue a long way, from red to blue for example, the new colour can look oddly glowing or oddly heavy. Fix it with Luminance: lower it after moving towards yellow or green, raise it after moving towards blue or purple. Small hue moves, such as nudging an orange-ish red into a cleaner red, rarely need any correction at all.