A protected ranking lets a player who has been out injured for a long spell enter tournaments using their old ranking rather than the one they dropped to. The ATP calls it a protected ranking and the WTA a special ranking. Both limit how many events it can be used for.
Rankings are calculated from results over a rolling period, so a player who misses most of a year loses almost everything they had earned. Without protection, a former top ten player coming back from surgery would have to start in the smallest tournaments, which is a punishing route back for an injury they did not choose.
The rules set a minimum absence before a player qualifies, freeze the ranking at an average of where they stood around the time they stopped, and then cap how many tournaments they may enter on it and over what period. It gets them into main draws; it does not get them seeded.
That last point matters. A returning player entering on a protected ranking is unseeded, so they can draw one of the best players in the world in the first round. Comebacks therefore often look worse on paper than the player's form deserves.