Parameters are part of the interface of a subroutine.
They allow you to customize the behavior of a subroutine to adapt it to a particular situation. The thermostat has a parameter, namely the dial that is used to set the desired temperature. Suppose that we want to write a subroutine to print out such sequences. But the exact sequence it prints out depends on the starting value of N. So, the starting value of N would be a parameter to the subroutine. One of the terms in the sequence.