Rarely used in BeReal's authenticity-first environment, but when it appears it's usually pointing out something funny or unusual captured in the candid photo.
reaction meaning
As a reaction it signals 'look at this' or 'I see you' to a friend's BeReal post, often acknowledging something specific in the background.
friend context
Friends use it to point out something they spotted in each other's BeReal photos that the poster may not have noticed.