nag is a prefix used in most tagalog words to change its tense into present and past tense:
for example:
nag + luto(cook) [root word] = nag-luto [cooked]
nag + luluto (cook) = nag-luluto [cooking]
to make a verb change it's form into present tense, we usually write the first syllable twice ,then add the prefix "nag" like in "nag-luluto".