So when we talk about systems, we simply talk about objects or things of interest, things that we're studying. And everything outside of the system is called the surrounding. Now, when we talk about entropy, we represent entropy with the letter S and changes in entropy with delta S.
So according to the conservation of energy, delta S or the change in entropy of the system plus the change in entropy, the surrounding will give you the change in entropy of the universe. Okay? Now remember, the Universe is an isolated system. And according to the second law of thermodynamics, an isolated system cannot decrease in entropy.