Selling Calls and Puts

I watched few videos online trying to understand covered calls but still ended up slightly confused. When I sell a call I get paid a premium straight away and regardless of the stock hitting the strike price on the expiration date which will sell 100 of my shares I still keep the premium.

So say I buy 100 NVDIA shares at 100 AVG and sell 140$ calls that will expire in December. I will end up making the 4000 for my shares and whatever the premium was. So why do I have a tab with that said stock that keeps going up or down depending on the stock price?