Meaning
A triangle pointing left, usually in a square, the mirror image of the play button. It means reverse or go back, and also serves as a bold left arrow.
It is handy for “back” links, previous steps in a guide or pointing at something on the left of a photo: “◀️ Swipe back to see the before picture” It can also signal returning to an earlier topic in a chat. For a fast rewind, the double ⏪ is the closer match.