From what I know, it's where you use your starter for the first gym, then (let's say you are playing Ruby) you catch rock types only after you beat her, then use that team for the next gym. Then, you catch all new types after you beat that gym, to match the types that that gym uses, and so on. It is something competive players love, because at the end, you have boxes full of well-rounded types, rather than a level 100 Swampert.