I saw this icon on my Career Profile page and can't figure out what it means.
Unlike other stats on that page like level and time played, this thing never seems to change.
I don't think I've seen an in-game acknowledgement of it either. So, what does it mean?
That's an unused indicator of your MMR rank for an upcoming patch that will add a Competitive mode.
In Closed Beta, there was competitive matchmaking and as such that image you posted is the default image displayed on the Competitive Play screen.
Note that currently Overwatch does not have a competitive matchmaking system; this answer will be updated appropriately when it is added.
You can see the same format in this gif Blizzard provided showcasing what competitive ranking will look like.
