@jennythelib - You'll see them everywhere now that you know what they are! My grandfather always drove a Chevy pickup because he likes the way the tailgates in them are (or were) trunnion-mounted.
Apparently, other tailgates are mounted on hinges, like a door, and take tools and a good bit of time to remove. But a trunnion tailgate, like he had on his full-size Chevy, can be easily removed if you need to for some reason. And apparently Granddad often wanted to take his off.