This is the polite version of the Tagalog word for 'yes.'
The word po is often used when talking to someone much older than you.
Compare the following sentences:
I don't know. (casual)
Ewan ko po.
I don't know. (respectful)
Salamat sa tulong mo.
Thank you for your help. (casual)
Salamat po sa tulong ninyo.
Thanks to your help. (respectful)
Sometimes, ho is substituted for po.
Opo. = Oho.
Salamat po. = Salamat ho.