Just same as how Gmail notifies you whenever there is a new email arrived, even if you aren't looking at Gmail,
Sell.do system also alerts you whenever the CRM events happen. For example, a new lead received, a call have been missed etc. These notifications alert you about the event even if you are not looking at the browser.
Note: sell.do notifications will only work if you have your browser open running sell.do when you receive the event notification. You do not need to be looking at the browser, but it does need to be running.