Learn French: Elle a les cheveux bruns.

In English, "hair" (the hair on your head) is usually a singular, uncountable noun. In French, it is always plural: les cheveux. If you say le cheveu, you are talking about just one single strand of hair!

Why does French use les here instead of a possessive like "her" (ses)?

When talking about body parts, French usually uses the definite article (le, la, les) rather than possessive adjectives (mon, ton, son) as long as it is obvious whose body part it is. Since the sentence starts with Elle a..., it is perfectly clear we are talking about her own hair.

Could I say Elle a les cheveux marron since marron means brown?

You might occasionally hear people say that, but brun (or châtain for light brown) is the traditional and natural way to describe brown hair in French. Save marron for objects, animals, or eye color!

Why does bruns have an "s" at the end?

Adjectives in French must agree with the noun they describe. Because les cheveux is a masculine plural noun, the adjective brun takes an "s" to become bruns. Like almost all color adjectives in French, it is also placed directly after the noun.