What does "extension" mean in the Axiom of extension

The axiom states that two sets are equal if they have the same elements, i.e. they are equal in "extension" (scope, content), as opposed to equality in "intension" (meaning, concept). For example, the set of black US presidents is currently equal in extension to the set containing Barack Obama as a single element, but they are different in intension. The axiom of extension means that the set theory only deals with the content of sets, not with the concepts used to form them.