Next generation was a term used to describe the newest iteration of a technology.

Even when the warp five engine was newly introduced and the only one in Starfleet was being used by Enterprise NX-01, warp five engines were expected to be installed in the next generation of freighters. (ENT: "Fortunate Son")

In 2375, Kathryn Janeway described the quantum slipstream drive "the next generation of instellar propulsion. (VOY: "Timeless")

(DIS: "Kobayashi Maru")

(DIS: "The Examples")

(DIS: "...But to Connect")