According to the context, for the sentences to be complete we must add the missing adjectives in the blank spaces:
—Mi padre, Juan, tiene una hermana en Cuba. Ella es Sandra. Ellos tienen el pelo castaño.
—¿Cómo es Sandra?
—Es bonita.
Translation:
—My father, Juan, has a sister in Cuba. This is Sandra. They have brown hair.
—What is Sandra like?
—She is pretty.
What is an adjective?
An adjective can refer to a specific characteristic of the noun and they usually coincide in gender and number with the noun.
In the sentence "Ellos tienen el pelo castaño", "castaño" is an adjective that expresses one of the characteristics of the noun."Pretty" is a feminine and singular adjective that attributes a specific characteristics to the noun (Sandra).
Therefore, we can conclude that an adjective is the part of the sentence that qualifies the noun, and that provides additional information.
Learn more about adjectives here: link
