The pulse of a melody defines its beats, and rhythm comes from playing notes with a combination of durations and beats.
A note can be held for several beats by putting a number like 2 or 3
after the pitch. It can be held for a fraction of a beat by putting
a fraction like /2 or 2/3 after the pitch.
Consider the difference between these two examples. First, five notes, each one beat:
play "C C C C C"
And five notes with different beats:
play "C C/3 C/3 C/3 C2"
Silent beats are called "rests" and they are represented by a Z
instead of a pitch.
play "C C Z C"
See more examples on the reference page for rests.
A fraction of a beat can be written C1/3 or just C/3 for short.
If the fraction is one-half, C/2 can be further shortened to C/:
play "C/F/FFC/C/C/_E/E/F"
Playing three notes in two beats time is an interesting and distinctive
rhythmic pattern. It can be written by giving each note a 2/3 beat,
but it can also be written by preceding the three notes by (3:
play "C C (3 C C C C"