It deals with a reserved power, e.g. the power to declare war and defend the nation against domestic or foreign enemies; which is a reserved power that is held by Congress.
Indeed, before the Constitution was ratified, there was an intense debate between Federalist and anti-federalists. The latter wanted to make sure that the rights of the individual states would be respected. This clause was included for that purpose. It states that any power not expressly delegated by the states ot the central government are states’ powers.