In Spanish, adjectives agree in gender and number with the nouns they modify. For gender, adjectives typically have masculine forms (often ending in -o) and feminine forms (usually ending in -a). For example, "alto" (tall, masculine) becomes "alta" (tall, feminine).
In terms of number, adjectives change to reflect singular or plural nouns. For instance, "alto" becomes "altos" for masculine plural and "alta" becomes "altas" for feminine plural. Thus, the correct form of an adjective depends on both the gender and number of the noun.