We are looking in at an apparently quiet scene where the maid delivers a letter to her mistress, but in fact the painting is a boiling emotional turmoil: the mistress is in love with one who is not her husband, and the maid knows all about it. The two are exchanging glances even before the letter is opened: each knows that it is the sender, not the contents, which is significant. Vermeer exploited this theme elsewhere, in Lady Writing a Letter with her Maid. The  curtain and the containing walls make us feel like spies, watching a private moment.

On the floor next to the mistress is a laundry basket and some embroidery work, and in the doorway is a broom, representing her neglected wifely duties. Instead, the mistress has been playing music, a symbol for thoughts of love. Behind her is a painting of boats moving fast before a wind, with an open sky in view, representing her state of mind; next to her is a fireplace, symbolizing the passion of love, and painted thereon is a lion rampant -- her lover, the letter writer.