Low-energy servers are servers that are designed for lower levels of energy use. These designs can include different kinds of specialized processors, as well as different features for making energy use more efficient.
Techopedia explains Low-Energy Servers
The basic idea behind low-energy servers is that they can handle specific requests from a network with lower energy use and smaller energy spikes. These can be great for larger hardware systems, but many different types of low-energy equipment are also built into individual smartphones and mobile devices that need to be able to work "leaner" when not attached to a power source, not to mention quietly.
In part, the emergence of low-energy servers is part of a greater design to offer scalable and distributed systems. This is evident in the efforts of some companies to develop systems like "micro servers" and other distributed computing models for more muscular hardware operations, again, with a lower energy footprint per task.