In the Spanish language, what do direct objects (lo, la, los, and las) "tell" in a sentence?

I think the first answer is confusing direct object
pronouns with articles.

Definite articles:  el, la, los,
las  (definition:  the)
Indefinite articles:  un, una, unos, unas  
(definition:  a/an, some)

Direct object pronouns:  lo, la,
los, las  (definition:  it, them)
These are used to replace a noun. 

They match in gender and number with the replaced noun. 

Example:   I see a book.    Yo veo un libro.    un libro = masc, singular =
lo            I see it (the book).   Yo lo veo.

I see something
feminine singular it is "Yo la veo."   (I see it.)
The plurals become:  "Yo
los veo."   Yo las veo."   (I see them.  for both)