In this quiet, reflective, inward-looking painting, a table and curtain separate us from the well-dressed young woman reading a letter. The large green curtain may be intended as part of the scene, or it may be a trompe d'oeil to look like the painting has a physical curtain to cover it, as was popular in Dutch households at the time.
She is enclosed also by the horizontality of the curtain rail and the deep shadow at the top of the picture. This enclosure and separation provide intimacy: she is alone in an unguarded moment, unviolated by the intrusion of the viewer.
On the table is a dish of apples and peaches, reminding us of Eve's Fall. The window is open, representing her desire to escape her duties as a housewife. There is a red curtain pulled back from the window: red is the color of love. She reads with great attention -- perhaps the letter is from her illicit lover.
The woman's hair and sleeve show for the first time Vermeer's use of brilliant blobs of impasto paint to render higlights.