Starting from the 30, as you chose the example of FL300, moving to the right, the first line you hit is the lowest temperature the aircraft can operate at at this level, going straight down I get around -63C, although it's on a tablet so not the best resolution.
Keeping going across from the 30, and you get the the end of the box on the right, again going straight down I'm coming up with -15C. This is the highest temperature at which the aircraft can operate at this level.
Not really sure what you're not getting about this?