Postfix is just a different way of writing math. It is sometimes called "reverse polish notation". There is a good wikipedia article on it here.
The program can be run using cargo run.
Just pass a postfix expression and the result will be printed on the next line.
Example adding the numbers one and two:
$ cargo run
1 2 +
3
In infix (normal) notation, this looks like "1 + 2".
More complicated expression:
$ cargo run
1 2 + 3 + 6 *
36
In infix notation, this is "((1 + 2) + 3) * 6".
To close the program, you can type exit or just use Ctrl-C.