There were two main issues in this case.
First, there was the issue of whether the bank itself was constitutional. The Constitution does not explicitly give Congress the authority to create a bank. Therefore, one issue was whether there was something in the Constitution that at least implied that the Congress had the authority to create a bank.
Second, there was the issue of whether a state could tax an activity of the federal government. The bank was an activity of the federal government and Maryland wanted to tax it. Was that legal, or would that violate the supremacy of the federal government.