For lay people ...
The second law of thermodynamics states that the TOTAL entropy of an ISOLATED system cannot decrease.
Entropy is a measure of disorder, and an isolated system is one for which neither matter nor energy enter or exit. Open systems can exist within an isolated system, and in such local regions entropy can decrease, as long as that decrease is offset by a larger increase in the open system's surroundings, such that the TOTAL entropy of the ISOLATED system does not decrease.