In Halo Wars 2, Supply is your most basic, and useful resource. Without it, constructing almost everything other than upgrades is impossible. It’s used for training units, buildings bases, and using leader powers. So obviously you’re going to want Supply, and a lot of it.
Whether you’re playing as the rogue Covenant Brutes or the UNSC, the easiest and most reliable way to get Supply is to build either a Harvester or a Supply Pad (depending on who you picked). These buildings will slowly generate Supply over time. Both of these buildings can be upgraded using Power, the other critical resource in a normal game of Halo Wars 2. Upgrading Supply-generating buildings will increase the amount of Supply it generates over time.
However don’t stop at just one source of Supply. In order to succeed in Halo Wars 2, you need to find the right balance between buildings that train units, and buildings that generate resources. If you find that you’re running out of Supply, build more Harvesters or pads. If you’re floating, or not able to spend all of your resources, then you probably need to build more unit training buildings instead. Once you’ve filled up all of the nodes at your base, you probably need to go ahead and claim an outpost.
Finally, it is also possible to find Supply sources around the map which appear as glowing blue box-like objects. To collect it, just simply move a unit over to it, and then press X. That unit will then start to collect it until it runs dry.