This is the 78th most frequent Portuguese word.
um
"Um" - "A" or "One" (masculine, singular).
'Um' acts as an indefinite article signaling the presence of a certain boy.
Havia um menino na praça.
There was a boy in the square.
'Um' is used here as the indefinite article, indicating a specific but not previously mentioned book.
Eu li um livro interessante ontem.
I read an interesting book yesterday.
'Um' serves as an indefinite article referring to a nonspecific moment in time.