Nouns in Spanish are divided into genders: masculine and femenine

Adjectives describe nouns.  They have different forms to match or agree with the noun. 

• The masculine form of most adjectives ends in -o

• while the femenine form ends in -a

Adjectives also agree with nouns in number. 

• If the adjective is describing a plural noun, then you need to make the adjective plural as well.