If you do NOT have an outstanding revolving balance on your credit card, a grace period is the interest-free time period between the date of your purchase and when the purchase appears on your statement. For example, if you pay off your balance in full on June 1st and then buy an item on June 2nd, you will not be charged interest for the time period between June 2nd and your next statement.
If you carry a balance on your credit card from month to month, you do not have a grace period. You are charged interest immediately when you make a purchase. For example, if you have a balance of $500 and make a payment of $50 on June 1st, and buy an item for $50 on June 2nd, you will immediately be charged interest on that $50 purchase. This is because you have an outstanding balance of approximately $450 between June 2nd and your next statement.