I did some digging, reaction engines are jet engines that can transition into a rocket propelled mode. It could enable a single stage to orbit plane.

Within the atmosphere the jet engine will burn atmospheric oxygen which would theoretically reduce the amount of fuel needed to reach a certain delta-V. Once outside of the atmosphere the engine switches to a rocket propelled mode, usually by injecting stored oxygen into the engine in addition to jet fuel.

So far there hasn't been a working prototype of a reaction engine. A British company SABRE closed down due to lack of funding but it did prove several pieces of the reactive engine design could work independently.