The behavior confused me too. It means that if you are focused in the outline pane, and type, say, a "d", the focus moves to the next headline that contains a "d". I think it's a vestigial remnant of an attempt to have a more sophisticated (but complicated) way of navigating to a node.
In practice, if the focus shifted to the outline pane but you thought it was still in the body - the usual case for me - when you started typing the outline would start jumping around apparently without rhyme or reason.