Google's particular method, known as PageRank, determines this importance according to who links what. If a high-ranking site links to a low-ranking one, that site will be given lots of juice. If a low-ranking site links to something, it doesn't have as much relevance. The text of the link also is taken into account.
Google Juice is very important in Search Engine Optimization. It's also the cause of cursed things like comment spam, link exchanges, and link blogs. This is collectively called spamdexing.
Rick: Damn homie you need to hit up Scott he has that crazy Google Juice, he finds everything.
Steve: I hit up Scott and 45 seconds later he found the video link. WOW!!
Rick: HUGE! I told ya