Often understood as a measure of disorder, entropy is a measure of the number of ways a thermodynamic system can be arranged spontaneously. According to the second law of thermodynamics which states that any spontaneous process increases the disorder of the universe, the entropy of an isolated system can never decrease. For a system which is not isolated to decrease in entropy the environment must increase by the same amount.
Entropy is a state function, meaning it doesn’t depend on the pathway its system took to create it. Therefore, even though one can’t measure the entropy of reactants directly, they can be calculated alongside the change in entropy that occurs during a reaction.