Is it just one particular weapon or is it something that happens on different weapons?
+1 is an enchantment. There are other types of enchantment which are conditional, e.g.
The Shortsword of First Blood is an uncommon shortsword that deals more damage to enemies who haven't been injured yet
Break the Unbroken: Deal an additional 1d8 Damage Types Piercing damage to targets that still have all their hitpoints.
https://bg3.wiki/wiki/Shortsword_of_First_BloodSo this extra 1d8 only procs if the target is at full HP.
My party has gear which gives an advantage to hit or damage if they are in shadow and I have gear which does it if the enemy is in light.