In the Spanish language, what do direct objects (lo, la, los, and las) "tell" in a sentence?
Well, this is a very interesting question. It is vitally important for those of us who speak English as our first language to realise that Romance languages (Italian, French and Spanish etc.) are very different in lots of ways, but one of them is the way that objects have a gender: they are either masculine of feminine. Thus the direct objects that you have mentioned, and also the indirect objects that you haven't mentioned (uno, una, unos, unas) play an important role because they need to match up with the gender of the noun.
For example, we need to say: "La plaza esta en el centro de la cuidad." Note here how "La" goes with "plaza" and "el" goes with "centro" as "plaza" is feminine and "centro" is masculine. Making sure that the genders are matched this way is an important aspect of mastering this language.
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)
Actually, the direct objects say: IT or THEM