PostEaseThe short answer
A pinned post is a post manually fixed to the top of a profile, staying visible above everything posted afterward. Most platforms that support pinning allow one or a small number at a time. It gives an account a way to control the first thing a new visitor sees, independent of chronological order.
Reserve it for something still relevant
A pinned post left unchanged for months, pointing at an offer or announcement that has since ended, works against new visitors rather than helping them. The value of pinning comes from controlling a first impression deliberately, which means revisiting the choice periodically rather than pinning something once and forgetting it.
A good pinned post orients a new visitor
A pinned post is often the first thing someone unfamiliar with the account sees, so it works best as something that orients a new visitor: an introduction to what the account is about, a best-performing piece of evergreen content, or a current offer. An announcement aimed at an audience that already follows closely is a weaker choice.