The word comes from the metal type era, when the "kern" was the part of a letterform that extended beyond its body. In digital type, this physical constraint disappeared, but the optical problem it addressed remained. Some letter combinations — AV, To, We, LT — create visual gaps that the eye reads as breaks in the word. Kerning closes these gaps.
Most people cannot name kerning, but everyone responds to it. Poorly kerned type reads as careless — a subtle signal that the creator did not attend to the details. Well-kerned type is invisible in the best possible way: it allows the reader to absorb the words without the letterforms getting in the way.
In practice, most quality typefaces include built-in kerning pairs — hundreds of predefined spacing adjustments for common letter combinations. The designer's role is to enable these pairs, choose typefaces that include them, and apply additional manual kerning when setting display type at large sizes, where every optical irregularity is amplified.