weapons and armor question

[Lockey]

When I get armor or weapons, it often has something like -1 or +5 after it. What does that exactly mean?

Does it mean it is more durable?
Or does that mean it gives more (or less) armor or damage?

Example would be
Staff (Master)
fire 10-15 +5

Would I add that +5 to the damage. Which would mean it be 15-20 in damage. Or does that +5 mean it is just more durable. Or would it be more accurate?

Thanks