I came across some of them and had the same questions. I think it's an emoji so it might be one of several things. It probably means the user might expect to be contacted before accepting but it could also not accept people at all or its posts might be private. Bios or pinned posts might have more information. In any case people will explain when asked.
It's not a pro-tip but maybe @feditips might be more aware of mast-etiquette