What is a Not-Com? It is simple a url in the address bar of your browser which ends with a different word after the "dot" than "com."
See this page which has a list of all the available Not-Com's.
Why do you need to know this? I anticipate it will not be long before we start seeing url's such as fab.kitchen or goto.church or fixmy.computer or smellthat.coffee or speedy.delivery or any other imaginable, unique name. You will look at it and think "that must be a typo!"
You will try to put the name in the address bar by moving the "dot" and adding "com" only to find that does not work. You will be shaking your head in confusion.
So, just take a moment to read the list to familiarize yourself to save yourself some shaking-head syndrome in the future.
Or maybe you want to go buy one of these. Have it it!