A variation theme is the way in which child products differ, for example by color, size or quantity. The allowable variation themes differ by product category.
Sometimes you’re allowed to use two variation themes. For example, in Apparel you can choose the variation theme “SizeName-ColorName”, so you can list all of the colors and sizes of the same item on the same detail page. This allows the customer to select their color and size preference without having to search for the individual listings.
Whichever theme you choose, make sure that the child products ONLY differ in that way. And make sure you use the correct variation theme. So, don’t use the “color” variation theme to then list children which actually differ by material or size, or that differ by color AND material or size.