Kerning is the process of adjusting space between characters in a line of text with the purpose of improving their appearance, using their natural shape and slope. Without using the technique on a web page, certain combinations of letters (like WA or VA) would affect the readability of a text, impeding the smooth flow of an eye movement through a copy. It’s more important to apply kerning to a piece of text when working with the fonts of italic types and oversized typography used in the headlines.
Simply put, kerning is the amount of space between two letters or numbers. The technique is commonly used in cases when the default kerning doesn't look perfect for certain combinations of letters, and the need to apply manual adjustments arises. Kerning is a visual exercise rather than the actual distance between the two elements in line. It's all about the process of the proper adjustment of letters within a word so that they look aesthetically pleasing.
Like each piece of a puzzle, each letter has its unique size. Years ago, people used physical blocks for typing. Although the technique is no longer applied in the modern age, the fonts that we use today work in a similar way. Each letter of the alphabet is surrounded by an invisible box. Pretty often, such boxes add too much extra space between letters, which results in an ugly and difficult to read copy. That's why the boundaries of the letters need to overlap to create an even spacing that is easy for the perception.
Automatic kerning refers to the one that is being applied automatically by a program. Automatic kerning features two subtypes of kerning - metric and optical. In both types of the automatic kerning, the system is set to ask you to specify the font size to which the technique should be applied.
Manual kerning allows you to override automatic kerning and apply specific kerning value to a set pair of characters in a specified piece of text. When applied skillfully, manual kerning can give better results than automatic kerning. The spacing that is perceived as normal by a machine may seem to be too close for a user eye or vice versa.