Hello,

this is a Horizon-Class Railgun from the video game "Homeworld: Deserts of Kharak".
Railguns are slow-moving, hard-hitting units that can fire from a great distance. They take a long time to aim before each shot but when they finally shoot they deal heavy damage to any other unit.
Railguns don't have especially thick armor (thick armor would make them even slower) and have trouble aiming at small targets, this makes Light Attack Vehicles a good choice for taking railguns down.
Another great way to destroy railguns is using artillery cruisers which can fire even further than railguns and don't need a line of sight.

Enjoy!