Disney offers a "no-expiration" option for all of their tickets. This means, if you have unused days on your tickets (you bought 7 days, but only used 4), those remaining days on the ticket will not expire. Otherwise, you would just lose those unused days on your tickets, resulting in a loss of money. The no-expiration option allows you to "lock in" your ticket price.
Usually, getting the no-expiration option on a Disney ticket does not make sense. You plan your vacation for 4 days, you get a 4 day base ticket, and use all 4 days.
However, sometimes it makes sense to get the no-expiration option. As I noted, it allows you to "lock in" your ticket price. As you may know, Disney often raises the price of their tickets over the course of just one year, so you may choose to add the no-expiration option to a Disney ticket that includes more days than you'll need on your single vacation. This way, on your next Disney vacation, you've already paid for your tickets (using those un-used days from the non-expiring tickets) and can save hundreds of dollars on the vacation.