The article does the annoying thing of using a technical term without definining it.

An open-cycle gas turbine is the "simple" configuration that draws in air through a pressurization stage, into a combustion chamber for fuel combustion, producing high-pressure hot gases that drive a turbine and generate power, with exhaust gases released into the atmosphere.

In other words, there is no recovery of heat from the exhaust, it's basically an aircraft jet engine core mounted on the ground and connected to a generator. They aren't very efficient but they are compact, relatively cheap, and quick to spin up during peak demand times.