What does the second law of thermodynamics say about entropy?
The second law of Thermodynamics (along with the Clausius' inequality) asserts the principle of increase of Entropy.
Putting it in simple words, Entropy of an isolated system cannot decrease : well it's always on an increase.
Put it the other way, the universe evolves in such a manner that the total entropy of the universe always increases.
The second law of thermodynamics, assigns directionality to natural processes.
Why does a fruit ripe ? What causes a spontaneous chemical reaction to take place ? Why do we age ?
All these processes happen because there is some increase of entropy associated with these. Whereas, reverse processes (such as we don't get younger) do not take place naturally.
All of these have a directionality associated with them.
Such a directionality of all natural processes is asserted by the second law in terms of entropy.