As the example illustrates, @code{ly:make-moment n m} constructs a
duration of n/m, for example @code{ly:make-moment 1 8} is an eight
note
duration and @code{ly:make-moment 7 16} is the duration of seven
sixteenths notes.
I like Mats's gloss of "n/m" and it might help even further to add the
qualifier "of a whole note", so: "As the example illustrates,
@code{ly:make-moment n m} constructs a duration equal to n/m of a
whole note. For example, @code{ly:make-moment 1 8} constructs a
duration equal to one eighth-note (or 1/8 of a whole note) and
@code{ly:make-moment 7 16} constructs a duration equal to seven
sixteenth-notes (or 7/16 of a whole note)."