The Open Source Initiative defines "Open Source" like this:

Open source is a development method for software that harnesses the power of distributed peer review and transparency of process. The promise of open source is better quality, higher reliability, more flexibility, lower cost, and an end to predatory vendor lock-in.

A translation is probably in order. "Distributed peer review and transparency of process" just means that a ton of people from around the world can work on Open Source projects. There aren't any employees, just volunteers. Volunteers, that is, who are frequently incredibly skilled programmers. The result is software that's designed, written, and tested by many experts whose motivation is developing great applications, not augmenting a company's bottom line.

The second sentence simply says that, by developing computer applications according to the Open Source model, we should expect very cheap and very good software. In fact, this has been just the case. Perhaps the best known example is the Linux, the Open Source operating system that drives many of the web servers on the Internet. Other examples include OpenOffice, a full-featured office suite; GIMP, a powerful image processing tool; and Mozilla Firefox, the popular web browser.

For you, all of this boils down to free- or low-cost software that's just as good as - if not superior to - expensive, commercial software. Many of these packages are designed to assist users in putting up professional websites with little effort. SiteScriptors takes advantage of a suite of of these web applications to develop powerful websites, all for far less money and time than you would spend on a custom web designer.