\kern instruction good for and how is it defined?
I cannot find it in Knuth's book TeXbook.
A "kern" is a typographic term for a nonbreakable space between two items. These items are usually, but not exclusively, glyphs.
\kern is a TeX primitive that serves to insert nonbreakable space. E.g.,
a\kern3pt b inserts
3pt of nonbreakable space between
\kern may occur in either horizontal mode (to insert horizontal space) or vertical mode (to insert vertical space). Something that
\kern can do but which is not feasible in traditional, i.e., metal-based, typography is to set a negative kerning amount. E.g.,
\kern-1em deletes rather than inserts 1em of space.
I cannot find [the term] in Knuth's book TeXbook.
That's quite puzzling. The TeXbook's index states that
\kern occurs on pp. 10, 40, 66, 75, 87, 168, 256, 263, 280, 306, 389, 395-395, 416, 424, and 454-455. In addition, according to the book's index, the word "kerns" occurs on pp. 4, 66, 75, 95-97, 110, 157, 168, 280, 286, 306, 444, and 454-455.