Norton Antivirus Just Reported A “tracking Cookie” Detected On My Comput. What Is “cookie”? How Do I Treat It?

As I am browsing the internet on my computer now, I got the above message. What is cookie? How could a cookie be tracking my computer and what for?
The antivirus also reported that, it requires attention. How do I take care of it?
I even noticed that, my computer just became much slower than usual, could it be responsible? What should I do?