3D render engines process CGI materials, models, and textures into a viewable format so CGI artists can visualize, make lighting and composition adjustments, and perfect their CGI work prior to final rendering and export.
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3D rendering is the process of moving a computerized 3D model into its 2D visual form. For photographers, the best metaphor for this is taking a raw image into its final form - your post-processing would be the metaphoric render engine in that process.

The 3D scene in the 3D software comprises materials, textures, models, and lighting - the 3D render engine builds the output of all these in a final image.

There are many render engines on the market today. They all are software that helps build the 1s and 0s of computer language into the final image's pixels.

The PRO EDU CGI courses use either Octane or Redshift as the render engine.