https://docs.google.com/document/d/1XmZnV_n5FcTCj4unj4X_0ALy2DpIJkJqpL1CR2aQfxE/edit?usp=sharing
Quick summary:
1 - this terrain system allows for multiple pieces of intervening terrain (or models) to impact a shooting attack. A shooting attack that must pass through a barricade, through a small wall, and into a ruin would confer the defending unit an extremely good defensive bonus.
2 - rather than the current system of terrain keywords, terrain is split into two main categories and two main attributes, creating 4 types: Small Obstacles, Large Obstacles, Small Area, and Large Area.
3 - this terrain system is designed to be more permissive than the terrain system currently used in 9th edition - i.e. easier for defending models to claim more, and more useful, terrain bonuses. The general aim is to reduce the lethality of the game overall, and reduce instances where a small sliver of a firing model can spot a tiny speck of a target model and attack that target unit as if it were standing entirely out in the open. Additionally this system should allow for a wider array of models to interact with terrain in a way other than as just a hindrance - hopping on a motorcycle will no longer suddenly make it so a wall does not protect you at all from incoming fire, and a single tree will no longer represent an impenetrable barrier for an armored battle tank.
4 - finally, this terrain system is designed to be easier for players in a casual setting and event organizers to clearly and quickly define their terrain collection, regardless of what it might be made from. The rigid 3" and 5" height requirements for the two actually impactful terrain traits are gone, and additional rules have been added to make maneuvering around and over smaller "scatter" terrain features and area terrain features designated by a perimeter of objects such as a forest quicker and easier to manage.