That is a level indicator that allows you to know when the camera is held level by hand or tripod. I suspect the 6D is like the 1DX series and sequentially pushing the info button will cycle through the displays back to where you normally have it but check your owner's manual.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
