To make it simple for every one including a layman to understand; we can say that diversification is a risk management technique which mixes a wide variety of investments within a portfolio.
The benefit of this technique is that one is able to make money irrespective of the prevailing conditions in the market as by diversifying one is able to spread the risk and is able to latch onto a good amount of the profit, though it may not be the highest profit. You can check out the best mutual fund here