@mcc I really don't understand the point of this feature at all, because realistically, you're going to click on the video to see the description or comments section anyways, so it's kind of a useless feature IMO.

You have to go into your YouTube account settings > Playback and performance, and turn off the "Video previews" option to disable the autoplaying video previews. There isn't a way to disable it while logged out, but I assume a third-party browser extension out there can do that.