Server-side includes (SSI) can save you time and effort when developing a Web site. You create files that are inserted into your Web page by the server when the user accesses a Web page. The server, where the final Web site is stored, processes a command that you've inserted into your Web page.
Using server-side includes enables the Web server to place information dynamically into your Web pages. Since the processing of the included information happens on the server, you cannot view the file properly without loading it onto the server. Dreamweaver simulates the final appearance of your Web page by displaying the included information in the Document window. Also, Dreamweaver displays the server-side include when you preview your Web page in the browser.