As you enter search terms in suggest mode (which looks surprisingly familiar,) Google makes some educated guesses and offers some additional words to narrow your search.
Google offers this description:
What is Google Suggest?
As you type into the search box, Google Suggest guesses what you’re typing and offers suggestions in real time. This is similar to Google’s “Did you mean?” feature that offers alternative spellings for your query after you search, except that it works in real time. For example, if you type “bass,” Google Suggest might offer a list of refinements that include “bass fishing” or “bass guitar.” Similarly, if you type in only part of a word, like “progr,” Google Suggest might offer you refinements like “programming,” “programming languages,” “progesterone,” or “progressive.” You can choose one by scrolling up or down the list with the arrow keys or mouse.
After trying suggest on a few of my common search terms it seems like the suggestions could be useful for narrowing down searches, but I think the bigger advantage may be helping less savvy computer users understand the power of Google and get better, faster, more accurate results.
Check out Google Suggest at www.google.com/webhp?complete=1&hl=en