It's possibly (nay, probably) malware which is trying unsuccessfully to display ads on your screen. Try downloading the free version of Malwarebytes Anti-Malware, installing it and running a 'quick' scan (which, in practice, often takes over half an hour to complete). When you get to the end of the scan, click on the option to see the details and then on the option to remove whatever has been found:
https://www.malwarebytes.org/free/
(When installing the program, take care to deselect the option to take a trial of the 'PRO' version).