PN stands for Polish Notation and RPN Reverse Polish Notation.
For example, when expressing 1 + 2 using PN,
1 + 2 is + 1 2.
using RPN, 1 + 2 is + 1 2.
PN is similar to English language, while RPN is similar to Japanese language.
in English, the structure is S V something, where S is subject, V is verb.
in Japanese, the structure is S something V.
thus, + 1 2 means "add 1 and 2" that is the same as English word order,
you can read it as if it is English language.
in case of 1 2 +, you can read it as "１と２を足しなさい" that is the same as Japanese word order.