So, this feature has been available for some people for a while but has now apparently rolled out for everybody.
And what is it, I hear you ask!
When you upload your YouTube videos, you now have the option of adding 3 different thumbnails, and YouTube will show them randomly.
YouTube will track which video performs best, and after 2 weeks will decide on the winner!
At that point, YouTube will only show the winning thumbnail from then on, although you can change this if you wish.
And importantly, you can do this for your previous videos!
Click on the details for a particular video, and on the current thumbnail, you can select the 3 dots in the top corner, as indicated on the screenshot.
You can then add a further 2 variations of your thumbnail, and let YouTube run its test.
Ooh, I need to do this! I feel like I'm horrible at thumbnails, but this could help me figure out what works and then I can refine it from there.