The article is here, but for those who can't follow it:

Say your file is this string of letters:

aabbcc

And you try to download from the one guy in the world who has this file, but he goes offline before you finish it. With SET, they've developed a scheme where anyone with any kind of file that shares sections of bits with your file can be sources. Before you needed this one guy, but if you find people with these files:

bbgggeeeyyy and iiuucc

Because they have the code chunks you need, you can download it from them instead (and it doesn't matter what kinds of files are involved, only that the code chunks match.