Learn French: Elle se lave le visage avec de l'eau froide. - She washes her face with cold water.

se is a reflexive pronoun. In French, when someone performs an action on themselves, you use one of these pronouns right before the verb. Because she is washing herself, we use se.

Why does the sentence say le visage (the face) instead of son visage (her face)?

In French, you usually don't use possessive words like "my" or "her" for body parts when the sentence already has a reflexive pronoun. Since se already shows it is her own body, you just use the word for "the", which is le.

Why is it de l'eau instead of just l'eau?

de l' is used here to mean "some". When talking about an uncountable amount of something, like water, French uses these partitive articles. So avec de l'eau means "with some water".

Why does froide come after eau?

Unlike in English, most describing words (adjectives) in French come after the noun they describe. Also, because eau is a feminine word, the adjective takes its feminine form, ending in an "e" (froide).