Render
The process of generating a final image or sequence from a digital file, incorporating lighting and physical properties.
In practice
Rendering is the computation that turns a digital scene, with its models, materials, lights and camera, into finished images or frames. Designers render to show what has not been made yet: a chair in walnut and in oak, or a lamp glowing in a dim room. Quality trades off against time; a quick preview takes seconds, while a final frame with realistic reflections and soft shadows may take minutes or hours.
Often confused with Mockup
A render is the computed image itself; a mockup is a realistic presentation of a design in context, which may or may not be produced by rendering.
Try this
In any free 3D tool, render one simple object under a single light, then move the light and render again.