An image caption is the text underneath a photo, which usually either explains what the photo is, or has a 'caption' explaining the mood. For example, it could be photography of a beach and have a caption, 'Beautiful beach in Miami, Florida', or, it could have a 'selfie' of a family having fun on the beach with the caption 'Vacation was fun, I can't wait to go back!'
I hope this helps!
Olivia