Medium Plating is a technology that can be researched at the War Center in Tech Level 3. It costs 150 Food and 100 Nova, and gives Mech Factory units +1 armor and +1 pierce armor. Light Plating must be researched before it becomes available, and Medium Plating is a requirement for Hvy Plating.
The extra armor makes it harder for enemies to take down the player's Mechs, and makes it more likely that they'll make it back to the player's base to be repaired.
"The next level of Mech plating makes them more damage-resistant."