Mud-covered Walls
From WikiRaider
Mud-covered Walls are a new hiding place in Shadow of the Tomb Raider. They work similarly to tall grass and other plants in the previous games. Lara is able to hide here from Enemies like the ever present Trinity mercenaries or the Yaaxil and stealthily strike without being detected.
To effectively use Mud-covered Walls, Lara first has to use Mud Patches to cover herself in mud.
Staying close to mud-covered walls will conceal you, if you are camouflaged with mud.
Shadow of the Tomb Raider, On-screen info
Mud-covered Walls offer no protection from Trinity soldiers equipped with Night Vision Goggles (other than the protection normal walls already do).