At the top of page 283, Knuth is describing what happens when TeX is in vertical mode and it finds
\noindent: this command starts horizontal mode appending nothing to the horizontal list.
At the bottom of page 282 the effect of
\indent is described: it starts horizontal mode appending an empty box as wide as
\parindent (the width can also be negative).
These two descriptions are in Chapter 24, “Summary of Vertical Mode”.
On page 286 the effect of
\noindent when TeX is in horizontal mode is described: the effect is doing nothing at all. This is in Chapter 25, “Summary of Horizontal Mode”.
Similarly, the effect of
\noindent in math mode is described: nothing at all again (page 291). This is in Chapter 26, “Summary of Math Mode”.