Kerning is the adjustment of specific pairs of letters to improve spacing and fit. (It’s distinct from letterspacing, which affects all pairs.) Most fonts come with hundreds and sometimes thousands of kerning pairs inserted by the font designer.
Below, notice how kerning reduces the large gaps between certain letter pairs, making them consistent with the rest of the font.
Professional page-layout programs make it possible to manually kern letter pairs. For professional typographers, this is a mandatory skill, but for everyone else, the built-in kerning is adequate.