Imagine you have a markdown file.

You could "render" it to html with pandoc, then serve the html from disk by a static web server.

This would be "build time" html - it's html before it's read by the server.

Then you could setup a cgi script, php-script or an application server that converted markdown to html - then sent it to the client. This would be server-side rendering.

Finally, you could send some html+javascript to the client that fetch the raw markdown, then generates html on the client. That would be client side rendering.