Ring of strength
A +6 or -4 ring of strength affects the strength of the wearer. If the enchantment is negative, the ring is normally cursed, and it can cause stat zero if you are unable to remove it.
Desirability
Gaining a little more strength rarely ever has a very dramatic effect on your character's performances. It provides a slight increase to your accuracy and damage output, and it allows you to wear armour, kite shields, and tower shields more effectively, but its overall impact is usually pretty negligible. A strength ring might be a good find for early characters wearing heavy armour or a shield, particularly if they also intend to cast spells, but you'll probably want to replace this with something better eventually.
History
Prior to 0.25, enchantment values ranged from 2 to 6.