Aleatoric is music in which some element of the composition is left to chance.
1) Choose six notes from the piano.
2) Then give each note a number.
3) Roll a dice any number of times to create a row of notes (tone row).
Here's an example of what might happen...
C = 1 F = 2 G = 3 G# = 4 A = 5 A# = 6
I've rolled the dice 5 time and had these numbers
3 then 4 then 2 then 2 then 6
G G# F F A# (my tone row)
Then I can make a melody with those notes!