The Tagalog abbreviation n.u. is short for ng umaga.

ng umaga
"of the morning" = in the morning

This is the equivalent of the English a.m. or AM (ante meridiem).

6:00 n.u. = 6:00 am
alas-sais ng umaga
six o'clock in the morning

10:00 n.u. = 10:00 am
alas-diyes ng umaga
ten o'clock in the morning

This Tagalog abbreviation is taught in schools but is rarely used.
Filipinos prefer to use the English A.M.