Entropy isn't easy to describe. You need to go through a beginning thermodynamics book mostly. However, that's never stopped me from posting, so here's a short description.
In any heat engine (automobile, for example) one does work by absorbing energy at one temperature (the combustion in the cylinder) and discharging heat at another (the exhaust, the cooling mainfold.) Some of this energy is converted to work (pushing the piston) but the rest is "waste" (from the point of the automobile.
The first law of thermodynamics basically says that the engine won't work if no energy is put into it (it's out of gas, for example.)
The second law says that all of the energy in the gas cannot be extracted to do work. Some of the energy must be wasted. Entropy is a variable that gives a quantitative measure of how much energy will be available or wasted.
Mathematically, one gets the equation: Free Energy (two types, named for Gibbs and Helmholtz; this is what's available for usefule work) equals the total energy input (enthalpy to use the technical term) minus the temperature times the entropy or: G=H-T*S. Entropy is a variable that has the same ontological status as heat or temperature.
You might surf the net looking for a popular explanation of the "Carnot cycle."
Thanks, Doc!