The statement "[Chief Justice] John Marshall has made his decision, now let him enforce it!" reflects the idea that the Supreme Court, while having the authority to interpret the law, lacks the power to enforce its decisions. Therefore, the best explanation of the meaning of this statement is:
The Supreme Court does not have the power to enforce its decision, so President Jackson could ignore it.
This implies that Jackson believed that it was up to him and the executive branch to enforce laws, and if he chose not to enforce the Court's ruling, the decision would effectively be rendered powerless.