Kerning refers to the spacing between the characters of a font. Without kerning, each character takes up a block of space and the next character is printed after it. When kerning is applied to a font, the characters can vertically overlap. This does not mean that the characters actually touch, but instead it allows part of two characters to take up the same vertical space.
For example, when the characters A and V are placed next to each other, they can take up less total space if they overlap. This is because the right part of the A and the left part of the V fit together. If kerning is applied to the two characters, you could draw a vertical line straight down starting from the top left part of the V and it would go through the lower right part of the A.
Kerning is useful because it allows more text to be placed within a given amount of space. This allows longer articles to be placed in newspapers and magazines with limited space. It also looks more natural because when writing by hand, people often make characters overlap. Many text editing programs, as well as image editors such as Adobe Photoshop, allow the user to kern characters. These programs often include a kerning setting that enables the user to determine how tightly the characters fit together.