It's a catch-all for the tech capabilities of a civilization. Expressed in terms of energy harvesting.

Eg. capturing all energy output of a star, includes the ability to move (vehicles, robots etc) through space, build star-sized infrastructure, and so on.

Personally I prefer the magnitude of energy production/consumption. MWatt → GW → TW etc.

Or how advanced (or energy-dense) its source is:

Burning stuff → electric power → nuclear fission → nuclear fusion → antimatter