The Articles of Confederation was a series of documents created by the Continental Congress declaring the new Union. It was basically the first constitution before the constitution we know today.
Issues leading up to Shays' Rebellion were mainly due to money. The Articles of Confederation weren't going smoothly and asking people like farmers for too many taxes. This caused previous war veterans, including Daniel Shays, to become angry because they didn't want to end up in debtor's prison.
Eventually, these people get into groups and protest against this. Massachusetts government ignored the farmers. Shays’ Rebellion was so important for the new U.S. because if there wasn’t a Rebellion to fight for a better Constitution, we would still have this awfully thought out constitution today and it would continue to cause a lot of people to go bankrupt.