Entropy is an abstract concept and is difficult to understand.
To put it in simple words,
"It is a degree of randomness of the system."
Let's understand the meaning of it.
Suppose we have a pure crystalline substance at 0 Kelvin. It has never seen heat. Never had its electrons gain any energy from outside.
Let's call it the best possible state a pure crystalline substance can have. (Obviously, this is all in imagination)
Now some heat is added from outside to the system and that heat is gained by electrons which results in an increase in the kinetic energy of electrons. These electrons start to vibrate and start undergoing all other kinds of motion in different directions.
Before gaining the heat we could easily define the positions of electrons. But after they gain some amount of heat and start to vibrate and move, can we define the position of electrons now? You might say yes. Let's add some more heat now they gain even more kinetic energy can we define the position of each electron now?
Can we predict the next position of the electron?
With more and more energy interactions, it becomes more and more difficult to define the position of the electrons. As we can not define the position of electrons we can not predict where the electrons will be at the next moment which is also known as uncertainty or randomness.
The fact that nobody can figure out what happens in the future is nothing but the result of randomness and uncertainty.
Now, to calculate how much the randomness has been increased because of the given process, we have defined a quantity called entropy. We say that entropy is zero at absolute zero temperature (0 Kelvin). We only calculate the change of entropy during a given process. The more the entropy changes during a given process the more randomness it creates and it becomes more and more difficult to define the position of electrons and in turn predict what would happen next.
That's entropy in simple words!