String Savers (the Prince version of string-a-lings) are little plactic pieces that are inserted where the strings touch around the sweetspot of the racquet. They prevent your strings from rubbing against each other, and are intended to help prolong string life. String Savers are legal because the ball never actually touchs them.
String Savers are mostly used by players that have gut strings. They are usually effective at prolonging string life -- but how much will vary. Some players complain that they fall out, particularly if they hit with a lot of spin. Some players also do not like String Savers because the give the racquet a stiffer feel.
Another alternative to lengthen string life is to use a thicker guage string. In any event, it is still recommended to replace your strings at least as many times in a year as you play on average in a week.
There is a little picture on the String Savers box showing how to install them, along with a tool for insertion. One box includes enough for one racquet.