Mary takes a pound of costly nard, anoints Jesus' feet, and wipes them with her hair until the house is filled with the fragrance. Judas objects to the expense and invokes the needs of the poor.
The scene contrasts calculated value with lavish honor.
Nard was an imported aromatic perfume associated with luxury and could command a very high price. John's 'pound' is a Roman litra, roughly three-quarters of a modern pound by weight. Anointing could express hospitality and honor, while Mary's attention to Jesus' feet places her in a strikingly humble posture. Letting down or using her hair in this intimate public action adds to the unusual character of the gesture.
Jesus connects the anointing with the day of his burial and echoes Deuteronomy 15 when speaking of the continuing presence of the poor.
The economic and social background explains why the room reacts to the act as extravagant. Mary uses something genuinely precious rather than a token amount of inexpensive fragrance.
Jesus does not dismiss care for the poor; the scriptural background of his statement assumes that responsibility. He identifies this particular moment as uniquely appropriate honor in view of his approaching death.