On MacBook and MacBook Pro computers, Mac OS X provides a feature that enables the trackpad to accept a "tap" as a mouse–click. You may find that if this feature is enabled while you type, your palms or thumbs may accidentally touch the trackpad, moving the cursor unexpectedly or creating a mouse–click in the wrong place.
To avoid this situation, you can enable the "Ignore accidental trackpad input" option, found in the Trackpad pane of System Preferences. You can open this pane by choosing System Preferences from the Apple () menu in the upper-left of your menu bar, then choosing Trackpad (or Mouse & Keyboard in Mac OS X v10.5 Leopard) from the View menu.
When you check the "Ignore accidental trackpad input" option, the trackpad will ignore any contact on the left or right edges of the trackpad while you are using the keyboard (or just after). You may notice that while you are typing, the cursor arrow will disappear from the screen and reappear when you move your finger over the middle part of the trackpad. If you try to move the cursor by touching the left or right edge of the trackpad first, the cursor may appear not to respond. Make sure you put your finger on the center of the trackpad to avoid this.
If you do not want to use the "Ignore accidental trackpad input" feature, you can simply disable it in the Trackpad pane of System Preferences. The Trackpad pane also includes controls for two-finger scrolling, tap, double-tap and dragging capabilities.