It can display images in your terminal. This is made pretty clear by the first paragraph on the page and also by the Wikipedia article it links to[0]; I'm wondering what else you're asking for.
Sixel is a very old standard and has been used to display graphics in terminals since the 80s. It fell out of favor when high-resolution bitmapped desktop environments became mainstream (and affordable), but it is still useful if you are doing remote console work and don't want to set up network hoops with ad hoc HTTP servers just to view a remote diagram.
(I define it as "written words in a human-readable language". You must have something else in mind, because neither ASCII art nor control codes fit my definition.)