What is rendering?

In short you can say rendering is the process of generating an image from a 2D or 3D model by computer programs.

In 3-D graphic design, rendering is the process of add shading, color and amination to a 2-D or 3-D wireframe in order to create life-like images on a screen. Rendering may be done ahead of time (pre-rendering) or it can be done in on-the-fly in real time. Real-time rendering is often used for 3-D video games, which require a high level of interactivity with the player. Pre-rendering, which is CPU-intensive but can be used to create more realistic images, is typically used for movie creation.