If your gift certificate is only good for a limited amount of time, you have a specific number of days in which you can use it. Limited means short, small, or restricted by specific rules.
When a rock concert has a limited number of seats, you may not be able to count on sitting down for the show. Likewise, it's tricky to open a business in a city with a limited amount of retail space. The adjective limited, which comes from the verb limit, was originally used to describe certain train routes — a limited was a term for an express train the late 1800's.