It's a poetic and metaphorical expression used to describe:
* Something beautiful that is fading or dying: This could refer to a literal flower, a person's beauty, a relationship, or even a moment in time.
* A loss of vitality or freshness: It can describe the feeling of sadness, loss, or disappointment.
* The transience of life: It emphasizes that beauty and joy are fleeting and nothing lasts forever.
In short, "flor que se marchita" is a poignant reminder of the ephemeral nature of life and the inevitability of change.