You have to reach 10.000 views on one video. Then you get invited to put advertisement on your video, and earn per view (you can add it to all videos, even new ones with zero views, as long as one video reached 10.000 )
For me it happened cause someone shared a link/embeded my video on a Japanese website, thus something out of my control, but it worked
I have yet to get enough views to start earning, but in the past, on a different webiste, I did earn quite a bit (but they closed the earning system)