The render path is the imaginary signal path inside the Ai software. It begins e.g. with a video clip or some generated stuff which makes up for a texture, and - after numerous stages - ends in a rendered GL output which is then sent to an output.

Most modules which have a yellow GL render port need to be hooked up to the render path even if they seemingly do not affect the actual video. You can make up for a render path either by connecting the video port to a GL window, or you hook it into the main render path with a Render Merge module. See an example here: deployment

Example - patch hooked into main render path with a Render Merge module: