PostEaseThe short answer
A call to action, often shortened to CTA, is a specific instruction telling a reader or viewer what to do next: comment, follow the link in bio, sign up, share with a friend. It exists because people often need a direct prompt to take an action they might otherwise have taken anyway, or might not think to take at all.
Specific and low-friction beats vague and demanding
A specific, low-effort action ("tell us your favorite in the comments") tends to convert better than a vague or demanding one ("engage with this post"). The more concrete the requested action, the more likely someone follows through.
One clear call to action beats several competing ones
A post asking for a like, a comment, a share, and a link click all at once tends to produce less of each than a post asking for just one specific action. Competing calls to action dilute each other rather than adding up.