A Dyson sphere is a megastructure that comprises a shell surrounding a star that captures the entire energy output of the star.
A single Dyson sphere therefore has an energy output equivalent to that of an entire Type 2.0 Civilization - one way a civilization can reach Type 2 on the Kardashev scale is to construct a Dyson sphere.
A Dyson sphere with a single purpose (rather than just energy collection) is called a stellar engine. Examples of these include Matrioshka brains, which use the energy output of the star for computation, and Shkadov thrusters, which use the energy for propulsion. Stellar engines are often constructed by Type 3.0 Civilizations and above.