The purchase APR on a credit card tells you how much more expensive the items that you charge to your card will become over the course of a year if you carry a balance from month to month. In other words, it’s the rate at which a balance from purchases will accrue interest on an annualized basis if you don’t pay your bill in full by the due date.
For example, if you buy something for $1,000 with a credit card that has a purchase APR of 10%, and you pay for $500 of it by the due date, the remaining $500 balance would grow to $550 in 12 months if you don’t pay it off before then. Credit card interest is actually assessed daily, though. That means your effective interest rate is the purchase APR divided by 365 (days in a year), and the interest you’re charged one day becomes part of the balance accruing interest the next. So the interest charges accrued in that example would actually be a bit more than $50 as a result.