The full text of the amendment reads as follows: The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.
They implemented this into the constitution in order to give more power to the states/people in creating and executing laws. Each state is needed to follow a certain set of rules and regulations. The state is not allowed to create laws that change or remove those sets of rules. However the state IS able to create laws outside of that, like school districts, housing, marriage, etc.