A very important factor in how your website is ranked in search engines is how many other websites link to it, and how highly-ranking and relevant those linking websites are. One popular way of generating these links is to become a "guest blogger" on websites that you would like to link to your website. Matt explains that there is a big difference between writing a piece of high-quality work as a guest blogger for a single website, and having the same piece (or variations thereof) of poor quality work republished on hundreds of other websites.
That is, if you are guest blogging in an honest and legitimate way, you don't have anything to worry about. But if you are guest blogging en masse without much consideration for the quality of your work or the quality of the websites you are being published on, be prepared for this to impact on your search engine rankings.