Staging

Throwing away the empty part of the rocket so you stop accelerating it.

A rocket spends most of its climb carrying tanks that are already empty, and the rocket equation punishes every kilogram of that. Staging drops the spent section — structure, engines and all — so the remaining vehicle is lighter and its engines can be optimised for the thinner air above. Almost nothing reaches orbit in a single stage; the margins are so tight that a single-stage-to-orbit vehicle would arrive with essentially no payload. Hot staging, which the Soviets used and Starship has revived, lights the upper stage before separation to avoid the moment of zero thrust in between.