A good real world example is fluid flow such as a gas compressed through a pipe, say a trumpet or an exhaust. If the standing wave in the pipe isn't optimal at the exit of the pipe a part of the energy will end up reflected which causes a very real obstruction and higher pressure. In a trumpet this manifests as a resonant filter that will leave all but a couple of frequencies without significant damping and that is the note (+ overtones) that you hear. In an exhaust such tuning will result in the system performing optimally at certain RPMs, and if you match that to when you want the engine to produce maximum power you can create a peak by get rid of the exhaust gases at that peak. Hence resonant exhaust systems tuned to the theoretical peak of an engine.