Marchitó is the past tense of the verb marchitar, which means 'to wither' or 'to fade'. It's a beautiful, poetic word that you won't often hear in everyday conversation.
In this iconic song, Selena uses it as the central metaphor. She sings that the love he gave her, "como la flor... se marchitó" (like the flower... it withered), perfectly capturing the feeling of a beautiful love that has sadly faded away.
This is one of Selena's most famous songs, and it's a perfect example of how to handle a breakup with class, even when you're falling apart inside. The singer knows her ex has already found someone new. Instead of being angry, she says, "Si en mi no encontraste felicidad / Tal vez alguien más te la dará" [If you didn't find happiness in me / Maybe someone else will give it to you]. It's a beautiful, if heartbreaking, thing to say to someone who left you.
But the song's main idea is in the title line: "Como la flor / Con tanto amor / Me diste tú, se marchito" [Like the flower / with so much love / you gave me, it withered]. The love that was once alive and beautiful is now dead. She accepts this, saying "yo sé perder" [I know how to lose], but immediately follows it with the painful cry of "a-a-ay, cómo me duele" [oh, how it hurts]. She's putting on a brave face, but the pain of losing him is overwhelming.