HP Pavilion dv6 - what is that key?

There's a key on my HP pavilion dv6 laptop that apparently redefines all the keys on the keyboard. Sadly HP has seen fit to label it with an ideogram instead of something in any language so I can only describe it (and can't search for help anywhere since I don't know what the heck to call it; was HP founded by The Artist Formerly Known As Prince?). Anyway, here is what it looks like: The key is between the alt and ctrl keys on the right of the spacebar. it is square and black. The symbol is white. It is two parallel thin white lines, then a thick white rectangle above that, then another thin white line.

I *think* this is the key that, when I hit it accidentally, stops me from doing anything useful with the keyboard. Does anyone know if that's what this is, and how to reverse the damage? Rebooting works but I hate losing work. Is there some way to disable the stupid thing altogether?