Learn Spanish (Latin America): Unas niñas leen las cartas.

Unas is the feminine, plural form of the indefinite article. Because niñas (girls) is feminine and plural, we use unas to match it. In English, it translates to "some" or "a few".

Why is the word for "the" las here instead of los?

In Spanish, articles must match the gender and number of the noun they describe. Cartas (letters) is a feminine plural noun, so it needs the feminine plural definite article, which is las. If the noun were masculine and plural, you would use los.

Why does the verb leen end in -en?

Leen comes from the verb leer (to read). Because the subject doing the reading is unas niñas (which counts as "they"), we use the "they" form of the verb. For regular verbs that end in -er, the "they" form ends in -en.

Does cartas mean "letters" in the alphabet, or "letters" sent in the mail?

Cartas refers to letters that you write and send to someone, or sometimes playing cards. If the girls were reading the letters of the alphabet, the word would be letras instead.