Otro (other, another) is an indefinite adjective/pronoun. It must always agree in gender and number with the noun it modifies.

singular
otro (masculine)
otra (femenine)

plural
otros (masculine)
otras (femenine)

Let's look at some example sentences.

Este libro es mejor que el otro.
This book is better than the other one.

Luis tiene otra novia.
Luis has another girlfriend.

¿Oye otras voces?
Do you hear other voices?

Mi mamá compra otros chocolates.
My mom buys other chocolates.

Note:
You cannot use the indefinite article un(una) before otro!

Tengo otra idea.
I have another idea.

Also remember that in Spanish the plural form "otros" occurs before, not after, a number.

Tengo que hacer otras mil cosas.
I have to do a thousand another things.