It’s a new fonctionnality on the Search engine Yahoo. When you do a search, you will automatically be given suggestions based on what you have typed / you are typing. Thus, Yahoo Keywords suggest presents some similar short queries you are searching
Search suggest is not new (you have already Firefox’ extension to help you) but it is new on Yahoo.com. That’s a good point. I already have a Firefox plugin to suggest keywords with Google… But it is not necessary to have it if you prefer to use Yahoo.