Question

What is Entropy? and why can't we reverse it??

Answer 1

Entropy is not a process, but a a quantity that measures how "random" or "disordered" a system is.The Second Law of Thermodynamics says that entropy only ever increases spontaneously. Gas molecules spontaneously diffuse around a room from a bottle (a spontaneous increase in entropy), but you won't see them neatly lining up and entering the same bottle again, just by themselves. That would be a spontaneous decrease in entropy. 

BUT, if you put energy in, you can certainly decrease entropy. For example, you can use a very powerful suction -- a huge "vacuum cleaner" -- to suck almost all the air molecules out of a room into a small chamber, which results in a huge decrease in entropy.

So what is IMpossible is a reversal of the "entropy only increases spontaneously" rule. But non-spontaneous processes, which require energy input, result in entropy decreases all the time.