You may have been advised to trash your Preferences. This is a small file called a .plist that is located in your user / Preferences folder. Navigate to ~/Library/Preferences/ folder and delete the file named com.apple.Aperture.plist.

(On Mac OS X Lion or later, to open your Library folder, hold down the Option key while clicking the Go menu in the Finder)

Trashing this file will reset all preferences, as have been configured in Aperture’s preferences. Before trashing it, you may want to make note of your current settings, or even take a series of screenshots that you can look back at. Even if you don’t, the next time you launch Aperture after trashing the preferences, you should open Aperture’s preferences and see if settings are as you want them.

In Aperture, open menu Aperture > Preferences… to view them.