Interference refers to a game feature called planes (of existence). Mortals usually reside only in plane A (or, you can also call it the physical world, or the physical plane).
However, only a part of all imaginable living beings are native to plane A.
There are beings which can be native to plane B, or plane C, and exist only partially in plane A (our plane). In other words, these beings are not 100% physical matter which is present completely in our plane, and appear to us only as blurry images and vague shapes.
The important point to understand is: If a monster is "out of plane" in this manner, it will not absorb the damage you cause on it in entirety. The destructive interference means a part of the damage you caused was lost in "translation" between your plane (A) and monster's plane (in whatever planes it exists - it can exist on more than one, or more than two at once). There is also constructive interference, which is the opposite effect.
You recognize this effect in monsters by looking at them. if they look "wispy" and "shadowy", you're probably dealing with a "multi plane" monster, a monster that is halfway in your plane and in some other.
Plane system also creates possibility for a player to be pushed "out of plane," and if this happens, items and beings in normal plane will become blurry and wispy, and be replaced with a completely new items and monsters that appear with the plane shift. If this new plane is also the monster's native plane, there is no more destructive interference, the image is no longer blurry and it will receive the damage (as far as plane maths go) 100%.
If you've seen Stephen King's Langoliers, or Poltergeist the movie, you will quickly grok how this works.
Related lore: Spirit world.