Unknown 'overlay' on some icons/folders

[headsortails]

I have the following icon overlay (2 people in a square)appearing on some files that I have downloaded. What does it mean? And how do I remove it?

*Edit - I guessed it would indicate a shared folder/file and when I tried to delete it, it said something like 'this folder is shared'. I checked the shared properties and it isn't shared. I got around it by making a new folder and copying the contents over. Why would a file arrive 'shared'?