This would be genric words that go before or after the keywords to create a uniue domain that isn't taken.
For example, if your keywords are "blue widgets" but bluedwidgets.com is taken, you could add "4u" after, to create bluewidgets4u.com.
411 (in the US we dial 411 for information. 411 is now slang for info".
With a decent list, anyone should be able to get a variation of the prime keywords of their choice.