Thermo - heat

dynam - change

ics - of or relating to, the study of

Thermodynamics. Of or relating to how heat changes. Or slightly rephrased, the study of how energy flows from place to place.

Energy flow is what allows anything to do anything. Keyword being flow -- all the energy in the universe won't help you do anything if it has nowhere with relatively low energy to flow to. A waterwheel set up on a small rushing stream will turn, but the same waterwheel on a glassy smooth sea will not. Studying thermodynamics helps you discover where all the proverbial streams are and how to tell them apart from useless seas.

Thermodynamics' sister, physics, will tell you how things move and bounce around after being kicked off. Thermodynamics tells you what kicked them off in the first place.