Intervening is a fancy way of asking, "Is something between these two elements?" It's used for many rules, most often when shooting. Sometimes it's obvious, like in the scene below. It's clear that **the terrain is intervening** between the shooter and the target.
Sometimes you need to check intervening more precisely. Draw imaginary lines from one point of the shooter's base to all parts of the target base facing the shooter. Anything that at least one of these lines crosses is Intervening. In other words, draw a sort of cone from one base to the other and see if anything crosses it. The key is that **the shooter decides what is the point on its base to check intervening from.** Thanks to this, you can get a better shot, getting around the terrain.
It's enough for any part of the terrain to cross the lines to count as intervening. If it crosses all the lines, it's **wholly intervening**. Some rare rules call for this, but for most, it doesn't make a difference.
When the shooter is on a Vantage terrain (elevated position), use the same rules. Draw 3D lines from one point of the attacker's base to all facing points of the target base. Any terrain that crosses these lines is intervening.
The same Intervening rules apply when the target is higher up on a Vantage terrain.