Ardor coin consists of forgers and bundlers. Forgers validate new nodes on the forging chain while bundlers merge multiple child chains into a ChildChainBlock transaction that is stored on the parent chain.
Consider the child chains to be in Bitcoins, then Bitcoin child chains would be bundled by the Bitcoin bundler. Similarly, the child chains having IGNIS will be bundled into one transaction by the Ignis bundler. The forging chain then carries out the forging of all new nodes on it using ARDR.