Most likely you visited an infected web site and this popup window is trying to trick you into installing the virus.
Assuming that is true, exit the web browser, clear its cache and data.
Don't go back to that site.
There are liars, cheats, frauds, swindlers and goodness knows what else in the world, who, one-way or another, want to make money out of you without caring how they do it. You've just encountered some.