An algorithm is a finite set of steps that it it takes to accomplish a set goal. If we were using reactor and we wanted to get soundS to come out of a speaker, we would come up with an algorithm or a step b step list of actions that must be taken by the computer or the user to get to the end goal. there are many different ways to get to the same goal and each one has its goods and bads.
an example of an algorithm in reactor
create an ensemble
delete the "L" sand "r" ins
create an instrument with two outs
go inside the instrument and delete the "L" sand "r" ins
create a notepitch
create an fm op
connect the note pitch to the pitch of the fm op
create a gate
connect the gate to the amplitude of the fm op
connect the two combiners to the fm ops out
play sound