In modern times, "Po" is an honorific commonly used on statements directed towards elderly people or your work superiors… this is to make the statements sound more "pleasant" and "respectful".
Ex:
Sa'an ka ng galing? - Casual
Sa'an po kayo ng galing? - Respectful
Both of the statements above translate to the same exact thing, (Where have you been?), however it is "socially frowned upon" to use the first statement when conversing with elderly people or your work superiors.
So what does "Po" mean?
"Po" is a shortened version of "Po'on", and "Po'on" is also a shortened version of "Pangino'on".
All three of these terms mean the same thing, they all translate to "Lord".
Back in ancient times these terms were used as honorifics when speaking to the royals and nobles who were seen as "divine" or "semi-divine", however in modern times, the terms "Po'on" and "Pangino'on" are exclusively used on deities, while "Po" became a honorific, specifically used when conversing with elderly people and work superiors.
Is that why I sometimes see people from Philippines end something (even in English) with "poo"? Which is funny, because in English "poo" is something very different :)