Photo Stamp Meetup

Just to preface, this isn't a denial, but a simple question m(_ _)m I sometimes see it on 📒, but what is a "photo stamp meetup"? I tend to just lightly tap when I see friend's photos, but is there some kind of culture or rule regarding photo stamps? I'm not looking to actively socialize, and I'm still figuring things out while just enjoying redecorating and changing outfits, so I don't really get it...😅