This symbol keeps two names at the top of my chats list on WhatsApp.
This feature is called 'Pin Chat'. It pins your chat to the top most and the grey pin icon implies that 'that' chat is pinned. To unpin the chat from top , simply press and hold the chat in the CHATS screen. Now an option bar will show up on the title bar. Select the first icon to unpin the chat from top. Hope it helps.