As you type on a PowerBook or iBook keyboard, your thumbs or palms may hover over the trackpad, occasionally touching it briefly. This may have the unintended effect of moving the pointer while typing, or, if trackpad clicking is on, moving the insertion point to a different part of your document.
Selecting the "Ignore trackpad while typing" option can prevent unintended trackpad movements. When selected, trackpad clicks, drags, and very small movements are ignored as you use the built-in keyboard.
Even when this option is selected, modifier keys (such as Option and Command), keystrokes on an external keyboard, and deliberate trackpad movements are not ignored. The trackpad button is also unaffected.