If you saw the Tactics playtests you are already familiar with this. The rule will also be in Planetfall and is easily adaptable to Five Parsecs From Home.
Scatter is terrain that is small and individual such as a tree, boulder, crate or barrel. A typical piece of scatter is about big enough that a single figure can take cover behind it, but no larger. It is called this because it can be "scattered" around the gaming table. Usually at the end of setting up your terrain, you might do that to just add a little more visual flair. If you have a few wide areas with no terrain, a couple of trees or rocks can break it up visually and make things look nicer.
The Scatter Terrain rule is a way to take that process and make it actually a part of the game mechanics.
A figure immediately behind (and touching) a piece of scatter terrain is in cover if they are being fired on from the opposing side.
Additionally if the firer rolls a natural 6, the shot blows apart or collapses the cover. The target avoids harm but the scatter terrain is removed from play.
The rule is as it is because it is quick and easy: No additional roll is required and you just have to check for a natural 6.
If you feel it is a bit weird to penalise a 6, you can change it so it occurs if you roll exactly the target number (or even if you roll 1 below the target number or a natural 1).
You could also make it a saving throw instead if you prefer to go that route.