Chainling Monsters, much like Tuner Monsters, are primarily used for the Resolve Summon or Resolve Monsters. At least 1 Chainling is required to execute a Resolve Summon. Some Resolve Monsters need specific Chainlings like some Synchro Monsters need specific Tuners. A Chainling can only be Summoned as the card in a chain that equals its own Level, and because of this, Level 1 Chainlings can just be Normal Summoned. A Chainling's Summon is treated as a Normal Summon, but can be used on your opponent's turn.
Chainling Monsters, like Resolve Momnsters, will be featured in Yu-Gi-Oh! D².
Examples:
Player 1 activates Pot of Greed
Player 2 can Normal Summon a Level 2 Chainling
Player 1 activates Mystical Space Typhoon
Player 2 chains Magic Jammer
Player 1 can Normal Summon a Level 3 Chainling