Questions & Answers about Mi lavo il collo e il viso.
Why does the sentence use il collo (the neck) instead of il mio collo (my neck)?
In Italian, when you talk about daily routines involving body parts, you use the definite article (il, la, etc.) instead of a possessive word like 'my' or 'your'. Because you already used the reflexive pronoun mi, it is perfectly clear whose neck and face you are washing.
What exactly does mi do in this sentence?
Mi is a reflexive pronoun that means 'myself'. The dictionary verb here is lavarsi (to wash oneself). While in English we say 'I wash my face', Italian expresses this as 'I wash to myself the face'. The mi is required to show who is receiving the action.
Could I just say Lavo il collo e il viso without the mi?
You could, but it changes the meaning! If you drop the mi, it sounds like you are washing a neck and a face that belong to someone else, like a baby or a patient. To show you are washing your own body, you must include the reflexive pronoun.