Adding description to the default template

I installed the lightbox plugin and using it to it’s simplest form. User clicks on an image on a page and the image opens up in lightbox. I’m trying to get the image description from the media library to show up under the image in the lightbox. I thought this was a default option, but it’s not showing up. How do I get this to show up? Thanks in advance!

https://wordpress.org/plugins/simple-lightbox/