Optical kerning adjusts the spacing between adjacent characters based on their shapes. Some fonts include robust kern-pair specifications. However, when a font includes only minimal built-in kerning or none at all, or if you use two different typefaces or sizes in one or more words on a line, you may want to use the optical kerning option. (Adobe.com)

In the digital typefaces used today, the space between letters is controlled by a table of kerning pairs, which specify spaces between different letter combinations. (Princeton Architectural Press)

The amount of empty space between letter pairs 'wants' to be equal, and that's the goal of hand kerning your type. Your mind reads the words your eyes see. When there is visual and spatial continuity between all the objects (letters) it's easier to read, the better things look, and the more you can concentrate on the meaning of the words, rather than the minute 'visual hitches' that are created by poor kerning. (coolhomepages.com - Tim Donahue)