Think of the ALT button just as the computer's ALT or SHIFT buttons. It gives a new meaning to the rest of the buttons.
For example, when ALT button is pressed, the shutter button no longer takes a picture but activates a CHDK script. That's what you see scrolling on the screen, the dummy "default" script that does nothing.
As you set CHDK to auto load, all CHDK features are enabled at the moment you turn on the camera. At least the default ones (like battery indicator, etc). If you want to activate more options you need to enter the MENU by pressing (in the S5) the menu button.
Take a look at CHDK's manual:
CHDK/End-Users-Guide-AllBest50 - CHDK WikiHave fun!