A chain is a sequence of rules. When a chain is invoked, the contained rules are evaluated one-by-one until the criteria of a rule is met, and the rule specifies ACCEPT, DROP or QUEUE.
A chain usually represents a particular kind of traffic direction within a table. Please refer to the subsection of tables for more information.
A chain has a chain-wide policy, which is specified as a target. If none of the contained rules match, then a chain uses the chain-wide policy as the action associated with packets for that chain.
Some chains are predefined in a table.