Toggle to view image descriptions

I feel really stupid, but I have a thing on my site laboursocialist.org which lets you toggle a description of the images. I want to do this on another site but I can’t remember how I did it on the first site! I’ve got the same accessibility plugin, WP Accessibility, and I can’t see that I’ve done anything different. And they are using the same theme Twenty Sixteen.
Here’s a screenshot of the little thing I mean: and this is what happens when you click on it:

• This topic was modified 8 years, 6 months ago by ruthbillheimer.
• This topic was modified 8 years, 6 months ago by Andrew Nevins.