This case is important because it set up the idea that the federal government can do more or less whatever it wants. It set up a broad definition of the power of Congress under the Constitution.
Before this case, it was not clear what Congress was allowed to do. There are very specific things that the Constitution explicitly says Congress may do. These are called expressed powers. But are these the only things Congress may do? This is the question McCulloch decided.
In this case, the Supreme Court ruled that the "elastic clause" is what we should pay attention to when deciding what Congress can do. It says Congress may do anything "necessary and proper" to carry out its expressed powers. In this case, the Court said that this clause allowed Congress to set up the Bank of the United States even though the Constitution never says "Congress may set up a bank."
So by ruling in this way, the Court said that Congress can do whatever it wants as long as that is not prohibited by the Constitution. Big difference, right -- Congress can do anything unless the Constitution prohibits it rather than Congress can do only what the Constitution explicitly says it may do.
Mcculloch v Maryland is important for two reasons:
1. The Marshall Court ruled that thanks to the language of Article I Section 8 Clause 18 (the elastic clause) Congress can do what is "necessary and proper" in the carrying out of the other listed powers given in Article I Section 8.
The Court ruled that the Federal Government could set up a national bank, because a bank is necessary and proper for the carrying out of other financial powers given to the Congress (borrowing money, taxing, etc).
2. The Marshall Court also ruled that the state of Maryland (or any state) may not take actions that impede the exercises of power by the Federal Government. In this case it was the taxing of the Bank of the U.S. by the state of Maryland
The two parts of the McCulloch v. Maryland decision shaped the continued growth of the federal government through the use of implied powers.