A central concept of capitalism that seems to have got a sort of renaissance - everyone seems to be chanting this one more than before.
The idea is simply this: When you buy stuff, you show your "support" for the makers. You shouldn't buy crap - by buying bad products, you tell (implictly) the producers they're doing good job and you want them to prosper. If you don't buy crap, you signal them you don't want them to, and they should make a change for the better. So, the company that gets no money will die - step down, like a politician that won't get enough votes.
I have mostly seen this phrase used by those who speak of media and art. For example, by buying good computer games and avoiding burger games you'd encourage computer game studios to make good games. Of course, the makers of "burger games" get more votes because of aggressive marketing... game market does have its parallels in political campaigns!