Sometimes, you'll have some major sections of a page that you don't want to accidentally reposition while manipulating the rest of your page. This is common with background sections, where you'd like to be able to easily select foreground objects, without accidentally selecting the background too, or where you'd like another user to edit only specific certain objects.
To lock objects, just first select them, and then press the Lock button on the Arrange palette. You'll see their resize handles turn to small X's to remind you that those objects are locked.
Locked objects cannot be selected as a group. Instead, you have to click on them one at a time. To unlock those objects, just click the unlock button.