SSI stands for Server Side Includes.
It is a simple mechanism that lets the server interpret special tags before sending a page to the visitor. That makes it possible to reuse parts of a page or inject small dynamic elements without building a full application.
What SSI is useful for
- shared headers and footers
- repeated navigation blocks
- simple dynamic fragments
SSI is one of the easiest ways to add limited dynamism to otherwise static pages. If your designer asks whether SSI is active, the important question is whether the server is configured to parse the file types your project uses.