In proportional spacing, the tightening of white space between letters to create a visually appealing flow to the text. Due to the slanted vertical lines in two-letter combinations such as AW, WA and TA, kerning reduces the white space between them for a better appearance. See proportional spacing
Note the vertical red lines. Not only is there no additional white space between the letters, they actually overlap into each other's space. Kerning is not used on monospaced fonts.