I don't know why this happening... chrome and windows 8. Ads don't appear on my other computer.
Your computer got a virus. Lichess has no ads.
It's a malware addition on google chrome. To remove it, settings (three little bars), select settings, extensions. If any add-ons are present, remove them. If you still see ads, remove google chrome entirely and use firefox.
Look into an anit-malware program like malwarebytes. You should also run a scan with antivirus.