Wearing a ring of sustain ability gives you the sustain ability property, which means that your base attributes cannot go up or down. It is not a good idea to wear this ring for long periods, because it prevents your attributes from being increased through exercise, but if you lack poison resistance, it is useful to wear a ring of sustain ability while eating poisonous corpses to avoid the Strength loss.
You could use it to fix your stats if you abuse them a lot (bad idea). A similar effect can be had by saving and restoring every 800 moves.
A ring of sustain ability can be identified by dropping it down a sink (losing the ring in the process), or by wearing it and doing something which ought to change your attributes like eating a poisonous corpse without poison resistance, eating a tin of spinach, quaffing a non-cursed potion of gain ability, #sitting on a coaligned altar, or applying a cursed unicorn horn. Alternatively, zap a wand of enlightenment and look for the line "You have fixed abilities".