Once the criteria of a rule is met, the target of a rule specifies what to do with the matching packet/fragment. Note that there is no need to specify a target. A rule without a target is still useful because it still counts the number of packets/fragements meeting the criteria of the rule for statistics purposes.
There are four types of targets, and a rule can specify one or more of these targets. ACCEPT specifies that the packet/fragment be processed the way it should be. DROP specifies that the packet or fragment be deleted.
QUEUE is interesting. It specifies that that packet/fragment be forwarded to “user space”. This means a user space program (with less privileges) can process the packet/fragment.
RETURN defers the decision to the next rule of the same chain (that contains the current active rule).