A somewhat old-fashioned or literary term for a love letter. In modern casual speech, ラブレター is more common.
She kept an old love letter in her desk drawer.
In period dramas, scenes of delivering a love letter often appear.
Compound of 付け (tsuke, 'attaching' or 'sending') and 文 (fumi, 'letter'). The exact historical derivation is uncertain, but it has been used since the Heian period to refer to love letters.