ERP is basically all of the typical office functions put together in an integrated package. For example, an Inventory module to track items, an Order Entry module to allow customers to order those items, an Accounts Receivable system to track the invoices generated when you shipped items to the customer, etc. etc. It's all integrated so, for example, the inventory transactions and the invoicing transactions all feed to the General Ledger for financial reporting.
It's "great" because you can't really run a company without it unless you want to do it all on paper.