John Gill Commentary Luke 1:38

"And Mary said, Behold, the handmaid of the Lord; be it unto me according to thy word. And the angel departed from her." — Luke 1:38 (ASV)

And Mary said, behold the handmaid of the Lord,
&c.] In which words she expresses her obedience of faith; she owns herself to be the handmaid of the Lord, and desires to obey him, and be submissive to him as such; and tacitly acknowledges her meanness, and great unworthiness:

be it unto me according to your word ;
she assented to what the angel said should be unto her; she earnestly desired it might be, and firmly believed it would be; she set her "Amen" to the angel's message:

and the angel departed from her ;
to the heavenly regions from where he came; to his great Lord and master, that sent him; having dispatched the business he came about, and which he was accountable to him for.