Your MacOS 10.14 Mojave is infected with 3 viruses!

Macware is very annoying on my Mac all the day. when ever I hot any new thing in the browser a new active tab opens and alerts me with this page"Your MacOS 10.14 Mojave is infected with 3 viruses!". and my Mac need to be scanned!!!!!!!. what is this error/bug actually. I'm very much fed up with this tab by closing every time when I'm busy with my work. do you have any solution for this ????? please save..... I have so many expectation from your end.

thanks and regards