Password Assistant simply calls the Mac OS X Password Assistant dialog on demand. If you close the window, just choose File → New Window (CMD-N) to get it back (or reopen the program).
Password Assistant is an excellent password generator built into OS X. To use it, click on the key icon next to the password field in places such as Keychain Access (/Applications/Utilities), the Accounts pane of System Preferences, or the Set Master Password dialog box in the Security preference pane’s FileVault tab. If you’d rather access Password Assistant like an ordinary program, download Code Poetry’s free Password Assistant utility.