Hey Antonio,
The captions on portfolio page are sourced from excerpts. To set the excerpt, you need to fill in the „Excerpt“ box just below the main text area in the post editor mode; in case the box is not present, click on the „Screen Options“ tab in the top right corner of the screen and tick the „Excerpt“ option in the list.
I’ve looked through the theme manual and noticed that there’s no explicit information on this (indeed not very intuitive) topic. I will update the manual as well. Thanks for bringing this up!
Cheers,
Andrew / Satori Studio