Just as in StarCraft, many units require Energy for their abilities. Additionally, Remnants utilizes what I’m calling a “soft ammo” system that will treat Energy as weapon ammo, and will penalize units in various ways when they run out of ammunition.
One example of this is the Fabricators faction’s Junkslinger unit, which has 110 Energy. Each shot it fires at an opponent costs 5 Energy, and it has an area damage ability that costs 60 Energy. When a Junkslinger’s Energy is exhausted, it fires more slowly and its range is reduced. It’s still able to participate in combat, but it’ll be less valuable and more vulnerable.
The effects of Energy depletion will vary on a case-by-case basis. Some spellcaster type units will be virtually useless, as when a Protoss High Templar runs out of Energy. Some units will do less damage, or move more slowly, or any number of other things. The effects of Energy depletion on a unit will be part of how that unit is balanced, though generally Energy depletion will make the unit worse at its role in a meaningful way.
On top of all this, very few units regenerate Energy on their own, and instead rely on Generators and Power Nodes for this.