Inking in the world of non-digital tools is the process of using pens or brushes and black India ink to finalize pencil lines and make them ready for toning or coloring. The printing presses back in the early days of comics weren't capable of printing grayscale, so dark black ink lines were needed to reproduce the comic art.
In the digital world, we aren't restricted to simply black ink for our lines, and we have a multitude of tools we can use to make our finalized lines. There is an art to good inking that we should discuss briefly.
For more detailed information on inking techniques for comics, I recommend Pen and Ink by Comikers, and How to Draw Noir Comics: The Art and Technique of Visual Storytelling by Shawn Martinbrough.
Inking is the art of taking a rough pencil sketch and providing finalized edges, volume, and a sense of light and shadow...