In Spanish, adjectives change based on two main things: number and gender.
First, gender refers to whether a noun is masculine or feminine. Most Spanish nouns are either masculine, which usually ends in -o (like "niño," meaning boy), or feminine, which usually ends in -a (like "niña," meaning girl). When you use an adjective to describe a noun, you have to make sure the adjective matches the gender of the noun. For example, you would say "niño alto" (tall boy) for a boy, but "niña alta" (tall girl) for a girl. The adjective changes form based on the gender of the noun it describes.
Second, there's number, which refers to whether the noun is singular or plural. When a noun is singular, the adjective usually ends with an -o for masculine (like "alto") or an -a for feminine (like "alta"). When the noun is plural, you generally add -s or -es to the adjective. For example, if you have more than one boy, you say "niños altos" (tall boys), and for multiple girls, it would be "niñas altas" (tall girls).
So, in summary, in Spanish, adjectives must agree in number and gender with the nouns they describe, which means you have to pay attention to how you change the endings of the adjectives when talking about different people or things.