It is pretty unusual to see malware on a Mac. But, like the one that told you that you need to update Adobe Flash a few years ago, this one wants to install code that allows things like more commercials to run in your Internet browser. I’m pretty sure this only occurs if you are using Apple’s Safari browser and you are visiting sites like Youtube. This particular malware is pretty unsophisticated so has an extreme low success rate for most users.
Long story short, if you come across something like you see below don’t click on the OK button. Click on the image to see the whole story.