The major significance of the Erie Canal was that it connected the Great Lakes to the Atlantic Ocean at New York City. This gave the United States a major new water route for trade.
Before the opening of the Erie Canal, farmers and others in what was called the Northwest (Illinois, Ohio, etc) had no easy access to the markets of the East Coast. They had to send their goods down the Mississippi River to New Orleans and then up the Atlantic coast. This was, obviously, very time consuming and inconvenient.
When the canal was completed, the Northwest was connected to the East Coast. This greatly expanded the market for goods from the Northwest and made that region much more attractive for settlers.