In Maya, rendering refers to the process of creating bitmap images of your scene based on the various shading, lighting, and camera attributes that you set.
When rendering, Maya takes into account all of the various objects and scene attributes, and performs mathematical calculations to produce the final image or image sequence. Once you render a sequence of images, you can then play them back in sequence, producing an animation.
You can think of rendering as getting your work ready for display. It's the shading, the lighting, and the camera settings that you apply to a scene so that it looks good when rendering single images or an entire animation.