Open Interest

As i am a beginner to trading, i only trade stocks for the time being.  But, many times i get a pop-up message in my trading terminal that  Open Interest on a particular security  is increased. So, you will not be allowed to take any fresh positions.

I wondered, why is Open Interest emphasized upon so much and thought to learn what it is all about. I got some very useful info which are as follows:

 What is Open Interest?

Open Interest is the total number of outstanding contracts that are held by market participants at the end of the day.

It can also be defined as the total number of futures contracts or option contracts that have not yet been exercised (squared off), expired, or fulfilled by delivery.

Source 1 

Source 2