What does the people icon mean next to a mailbox in Outlook desktop?

I can't upload a screenshot but in my Outlook desktop, next to my mailbox name in the sidebar to the right of the .com, there is a small icon of two people, a black silhouette of a head and body and then a smaller silhouette to the right of it.

I've not seen this before, what does it mean? Is there something shared in my mailbox? Is it something to do with contacts? With shared Onedrive folders/files?