I suppose I could've explained it... It's ideal for keyboard-heavy users.
Instead of typing a search term into the search box and Ctrl+Arrow-ing to the search engine I want to use, with YubNub (in Firefox and Chrome, maybe IE?) you type in a command for what you want to do.
"g FDR" does a Google search of "FDR".
b Bing, y Yahoo, yt YouTube, CNN, ESPN, IMDB, and on and on.
There's even some slightly stronger commands:
"tr Chi Eng 晚安" uses Google Translate from Chinese to English, telling me "晚安" means "good night".
Yeah, I mostly use the "g" and "yt" commands, but I love the flexibility without having to arrange search engines. I just remember a few letters/commands. For a long time I'd resorted to typing "site:imdb.com" as my first search term into a Google search box. No more.
Also, you can define your own shortcuts! On a whim, I pointed XBLA to the Xbox Live marketplace at Xbox.com, and it works wonderfully. So now I can just type "xbla Trenched" and get to the page so I can buy the game with a click.
A few of their "Golden Egg" commands:
http://yubnub.org/kernel/golden_eggs?args=