In short, a "simple caption" refers to a short and straightforward description or explanation of something, typically accompanying an image or a video. Full definition
Toddlers love to look at homemade books, scrapbooks, or photo albums full of people they know (try adding simple captions). (kidshealth.org)
But the restaurant chain seems to be sticking with him, sending out a picture of him later in the week with a simple caption: «Tough crowd.» (canadianbusiness.com)
For me, a proud Canadian, a simple caption in a parenting magazine that stated that «4 million diapers are sent to landfill each day in Canada» was what started me out in my cloth diaper journey. (thinking-about-cloth-diapers.com)