It means two things:
1) Google doesn't show very many of the links it knows about using that command. Normally around 1%. If you log into webmaster tools, you can see a lot more - but only for verified sites (ie your own - not competitors). However most people think that this still isn't everything Google knows about - ie they don't want to tell you exactly how they are calculating the ranking. Using the same command in Yahoo will also give you a lot more.
2) When you type in a site name say
http://www.widgets.com.au , Google will list every result with the word widgets on it, and basically ignore the www and the .com.au bits. This means you will see some sites with yours listed on them, but also other sites that are nothing to do with yours and are not links to you at all. It's basically completely unreliable as a way of measuring anything at all. It's annoying that Google does this but nothing you can do about it that i'm aware of.
Best of luck
Matt
Marketing Web