Adjectives are used to add information about nouns and pronouns. They also must agree with the noun that they are describing in gender and also in number. For example the noun las casas (the houses) is feminine and plural, so if we add the adjective grandes (big) it must also be plural and feminine. Another important characteristic of adjectives is that, in Spanish, they usually come after the noun.
The majority of masculine singular adjectives that end in -o, have a feminine form by exchanging the -o for an -a, and their plural form adds an -s.