Someone pays you money to go to as many blogs as you can (preferably high-readership ones) and leave comments advertising a product/service/website etc. Usually they're completely irrelevant to the theme of the post and the combination of advertising, irrelevancy, and the fact that the person commenting is making no effort to become part of the "community" makes it particularly annoying to the blog owner, and the regular readers. Imagine a spam email copied and pasted and posted into the comments section of a blog rather than being in your email inbox.