Saying "po" and "opo" is a sign of respect when answering "yes" to an elder. This is one of the trademarks of Filipino culture. This is still prevalent in most Filipino households.
My brother's kids don't speak Tagalog because they were taught English as a primary language but can understand Tagalog a little bit. When we tell them about the use of "po" and "opo" there is a bit of confusion since the English language doesn't have an equivalent word for "po" and "opo".
Elderly: "Hijo, kumain ka na ba?" (Young man, did you eat already?)
Hijo: "Opo, kumain na po." (Yes, I already ate.)
The English language doesn't have an equivalent word of respect when saying "yes" or "no". In English, the sign of respect is always with the intonation and the manner of answering.
My nephew JJ then says:
JJ: "Ninang, why do I have to use "APA" when saying yes?"
Me: "It's not "APA", it's "opo" JJ, it's a sign of respect when you are talking to an elder."
JJ: "Opo?" "Do I have to say it?"
Me: "Yes, you say "po" and "opo" to an elder when you want to say "yes" in Tagalog."
JJ; "But Ninang, I don't want to speak Tagalog eh."
My nice Ashley, who's been intently listening butts in:
Ashley: "Why not Chinese?"