Eggy can give you the details of how the rule works. But basically, the rule states that if you're out injured for a long time (6 months+), you get a "protected ranking" equal to what your ranking was at the time of injury. This can be used to help you get entry into tournaments (although not for seeding purposes).
An example was this year's Eastbourne tournament. After Chanda Rubin withdrew from Charleston, she extended her injury absence to 6 months. This allowed her to use a protected ranking about 15 places above her current ranking to gain direct entry into the Eastbourne singles, without having to use a wildcard.
As for how long a protected ranking lasts, TheBoiledEgg can give you details on the current rules. If he doesn't show up here, Private Message him. I thought it was 6 months or 6 events. Not sure which.