ncreases durability
For most items, (100/(Level+1))% chance a use reduces durability. On average, lifetime is (Level+1) times as long.
For armor, (60 + (40/(Level+1)))% chance a use reduces durability. (In other words, each durability hit against “unbreaking” armor has a 20%/27%/30% chance of being ignored.) Thus, on average, armor lasts 25%/36%/43% longer.
When durability is reduced by multiple points (e.g. hooking a mob with a fishing rods or Thorns armor), Unbreaking is applied for each point of reduction.