A single source of truth means that for any given piece of information, such as a customer's address or the price of a product, there is one authoritative record. Other systems and reports read from it rather than keeping their own copies.
Without it, the sales sheet, the warehouse list and the accounts all say slightly different things, and meetings are spent arguing about whose number is right. With it, a correction is made once and appears everywhere. This is the central idea behind joining up operations, finance, sales and the inbox.