SHTML stands for Server-Side Include Hypertext Markup Language
SSI lets you embed a number of special 'commands' into the HTML itself. When the server reads an SSI document, it looks for these commands and performs the necessary action. For example, there is an SSI command which inserts the document's last modification time. When the server reads a file with this command in, it replaces the command with the appropriate time.
check this link below http://www.apacheweek.com/features/ssi