This is how Jakob Nielsen describes it in his Alertbox column:

Progressive disclosure defers advanced or rarely used features to a secondary screen, making applications easier to learn and less error-prone.

Progressive disclosure works wonderfully well with search engine interfaces, and Google has used this to its full advantage making it its trademark with its zen-like front page that only displays a search bar.

This also seems like an ideal solution for mobile handsets that are always suffering from increasing complexity but needs to fit all the functionality into a screen the size of a business card.

Not so fast. As with all seductive terms, one has to be cautious. It is not a silver bullet for UX woes. It does not provide license to forgo careful study of how users use devices and interfaces: what are the core set of features that should be displayed on the initial screen? How and when should the secondary screen with the advanced features be accessed?

Simplicity is always what interface designers strive for and user want. Simplicity is never simple, and certainly not an excuse to be lazy.