“Opo” translates to “yes” (its casual counterpart is just “oo”); “po” has no direct English word, it’s a politeness particle tacked onto a sentence the way English might add “sir” or “ma’am,” but used far more often and with less formality attached.
When you answer a question from an elder or a superior, end your sentence with “po” — for example, “Salamat po” (thank you) or “Opo” (yes, respectfully).
You don’t need to use it with friends or people your own age, but when in doubt with someone older, adding “po” is always the safer, more respectful choice.