A caption is the text posted alongside a video or photo: the sentence or two under a post that sets up what is about to play, asks a question, or carries the call to action the video itself does not say out loud.
On a platform where sound is often off until someone chooses to turn it on, the caption is doing work the opening seconds of the video cannot.
A caption is also where a call to action usually lives, and where the first line has to earn a second one being read at all.
A caption is also searchable in a way a spoken line is not, so the words chosen for it carry a second job beyond the hook: describing what the video is about in language an audience, or a platform's own search, would actually type. The first line usually matters most, since a platform often truncates the rest behind a tap to expand.
A common mistake is writing the caption as an afterthought once the video is finished, when it is doing real work: the first line either earns a second one being read, or gets scrolled past along with the video, so it carries nearly as much weight as the hook itself.
A caption also has to survive being read with the sound off and the video not yet watched, which is a different job from a spoken line inside the video — a caption that only makes sense after watching is not doing the job a caption exists to do. A caption that repeats the hook word for word wastes the extra space it has, and one that promises something the video does not deliver costs the account credibility on the post that follows it.
Where it comes from
Instagram's own Help Center documents writing one as a normal step before sharing a post.
Read the source: Instagram Help Center — Add a caption before sharing your post.
What Rankfield reads
The landing page answers this directly, in its own words:
Your account end to end, your competitors, and the wider market around you: posts, videos, captions, comments, rhythm, what you offer. It watches the videos, not just the numbers under them.