The Doom: Eternal demons have a variety of hellish weapons at their disposal to try and take you down. A boring defensive measure that some demons have access to is a shield. They either hold them in front of them or throw them from a distance to block you in a fight. Shields are difficult to remove and lowering them consumes a lot of your ammo. However, you have a useful weapon to overload it.
The best weapon to use against these shields is the Plasma Rifle, the third weapon in your arsenal. While it does a lot more damage against shields than other weapons, you have the option of overloading the shield. You can see this with the standard shields that the zombie soldiers hold in front of them as they shoot at you. When you shoot it continuously, the blue shield steadily turns purple, then darker, then turns red. If you keep hitting it with the Plasma Rifle while it's red, it will explode, causing an area of effect and damaging the zombie holding it, including any other demons nearby. It's a great way to destroy multiple opponents at once.
Other enemies you encounter in Doom: Eternal will also use a shield in combat, such as Hulk and Destiny Hunter. The Hulk sends out a shield to block your path or protect other demons you fight. While you can kill the Carcass fairly quickly, using your Plasma Rifle to damage other demons is also a great strategy, especially if you have challenges. When you fight a Doom Hunter for the first time, you always want to use the Plasma Rifle to lower its shield so that you can start damaging it with the chaingun or rocket launcher.
Overloading a demon's shield is a great strategy. There are a handful of challenges you can complete by doing this, so make sure you stay on the lookout for those shields and detonate them from other demons nearby.