Something is refundable when you are allowed to return it and get your money back. Go ahead and buy that refundable plane ticket — you can always change your mind!
Most goods are refundable, although sometimes you need to show your receipt to prove that you made the purchase. If you're spending a lot of money on something, you should make sure it's refundable first. Don't book a pricey hotel in France when you're not sure you can make the trip, unless you know you can get a refund! The Latin refundere, "give back," is the root of refundable.