Aperture includes a set of keyboard shortcuts that filter images by keyword so you can quickly see only those images that are tagged as, say, a "portrait." These shortcuts work the same way as those for sorting images by rating, and they have the same effect you'd get by using the Filter HUD.
The keyboard shortcut to filter by keyword is Command-number, where number is the number assigned to the desired keyword. For example, you could press Command-1 to display all images tagged with the keyword Action. If you don't have any images tagged as Action, Aperture won't display any images.
The keyword numbers, which are configurable, appear on the buttons in the keyword control bar. To learn more about assigning and using keywords, see "Applying Keywords to Images" on page 117 of the Aperture Getting Started guide.
To remove the keyword filter, you can just click the "x" in the search field to clear it. This will return you to the default view of "unrated or better" images. To see all images, including rejects: Press Control-` (accent grave, same key as tilde); or click the magnifying glass icon to access the Filter HUD, then deselect the keyword criterion.