The word pag has many meanings in Tagalog.
It could be short for the word kapag, meaning if, when, whenever or upon.
= Pag tumibok ang puso
When the heart beats
The syllable pag is also often attached in front of words to make them into a noun.
act of avoiding
Ang iyong pagsagot
Your act of answering
the act of drinking
ang pag-download ng software
the act of downloading software
There are many other ways the syllable pag can transform the meaning of Tagalog words.
to allow one to travel
Foreigners often encounter pag in the word pag-ibig (the noun for love). Examples of other common Tagalog words that start with pag: pagkakataon (opportunity), pagkakaiba (difference) and pag-asa (hope).