Adjectives in Spanish as in English modify a noun. In Spanish however, there are two very important differences.
1) Adjectives in Spanish must match the gender of the noun they modify.
The table is red. - El mesa es rubia.
Most adjectives that end in a consonant or a vowel other than "o", do not change gender. "Joven - young" does not change whether it is modifying a female or a male.
2) Adjectives in Spanish must agree in number to the noun they modify. In other words if the noun is plural the adjective will be plural.
The tables are red. Los mesas son rubias.
To form the plural of an adjective that ends in a vowel you add "s". "verde - verdes". To make the plural of an adjective that ends in consonant you add "es". "Joven - Jovenes".