Simply speaking, it means that a force (action) that influence on an object causes a counter force (reaction). In other words: forces on an object work in pairs. They have different directions in which they work, but equal magnitudes.
A jet is moving thanks to the jet engine that pushes it forwards. The force, which is needed to change the state of motion, is in the jet's direction (action). The jet in turn pushes the engine backwards, so in the counter direction with a counter force (reaction).
Strictly speaking all motions wouldn't work without these forces in pairs, even if it is just walking...