Learn Spanish (Latin America): Ella se lava las manos antes de comer.

In Spanish, when you talk about body parts or clothing, you usually use the definite article (el, la, los, las) instead of a possessive word like sus. Because the sentence already uses se to show she is doing the action to herself, it is already perfectly clear whose hands they are!

Why is manos paired with las if the word ends in '-o'?

Mano is one of the most famous exceptions to the rule that nouns ending in '-o' are masculine. It is actually a feminine word, which is why it takes feminine articles: la mano for the singular and las manos for the plural.

Why is se needed before lava?

The verb used here is lavarse (to wash oneself). The pronoun se shows that the action reflects back onto her. If you just said Ella lava, it would mean 'She washes' (like washing a car or clothes), but it wouldn't imply she is washing a part of her own body.

Why does the sentence use the infinitive comer instead of comiendo for 'eating'?