A chain is a pretty nifty object on a board. It can hold Matchables, Boosters, Collectables, or simply nothing. Chains never move, nor will any chained items until you break them free.

You can still match matchables held by the Chain. If the chain has a Light Bulb, the only way to break it is to break the chain to free the Light Bulb from an explosion. The same goes for power-ups.