A call to action is the line — spoken, written or on a button — that tells the audience exactly what to do next: "shop the sale", "save this for later", "comment your pick", "link in bio". A good CTA is specific, singular and matched to where the person is in their journey, from a soft "follow for more" early on to a direct "start free" when they are ready to buy.
It matters because attention rarely converts on its own. People scroll fast and act on instruction; a post with no clear next step leaks the interest it earned. On social, the CTA also shapes which engagement you get — asking for saves or shares can feed the ranking system, while asking for comments sparks conversation.
Use one CTA per post, make the value obvious ("save this checklist"), and put it where it will be seen — early in a caption, on the last frame of a video, or as the pinned first comment. Then measure click-through and action rate, not just likes.