1. Definite and Indefinite Articles
Definite articles [the in English] refer to specific things or people. Indefinite articles [a or an in English] refer to nonspecific things or people.
I want to read the book that you have.
[a specific book]
I want to read a book.
[any book – no book in particular]
Because articles are adjectives, in Spanish, Definite and Indefinite articles must agree with the noun they describe in gender and number.

2. In Spanish the Definite articles [‘the’] are el or la, depending upon the gender of the noun. [‘el’ is masculine and ‘la’ is feminine]
El colegio the high school
la escuela the school

3. In Spanish the Indefinite articles [‘a’ or ‘an’] are un or una.
Un colegio a high school
Una escuela a school
Un avión an airplane
una manzana an apple
Un and una can also mena one when used before a singular noun.
Tengo dos cuadernos y un lápiz.
I have two notebooks and one pencil.

4. The plural forms of el and la are los and las
The plural forms of el and la are los and las. The plural forms of un and una are unos and unas. Unos and unas are usually translated as some.
Los libros están en la mesa. The books are on the table.
Unos libros están en la mesa. Some books are on the table.
Las revistas están en la mesa. The magazines are on the table.
Unas revistas están en la mesa. Some magazines are on the table.