Marchita comes from the verb marchitar, meaning "to wither" or "to fade." It's a vivid and somewhat melancholic word often associated with flowers or plants losing their vitality.
In this song, the lyric "La rosa si no la cuidas, se marchita" (The rose, if you don't take care of it, withers) uses this word metaphorically. It implies that love or a relationship, much like a rose, will fade and die if it's not nurtured and cared for, adding a layer of emotional depth to the song's message about valuing a partner.
Nyno Vargas's "Ve Y Dile" is a passionate plea from a man who's fallen for a woman currently in an unhappy relationship. He sees her pain and believes she deserves so much better. He's not afraid to tell her current partner's shortcomings, saying things like "los veranos se hacen frío, no te trata bien" [summers turn cold, he doesn't treat you well].
He's confident that he can offer her the love and respect she's missing. He wants her to realize, "que ya te cansaste confiesale, que mereces algo mejor que el" [confess that you're tired, that you deserve something better than him]. Even though she's been hurt and is hesitant to trust again, saying she doesn't want to know about love, he's patient and determined to win her over, promising, "por ti yo daría la vida, déjame sanar tu herida" [for you I'd give my life, let me heal your wound]. He encourages her to fly free, like an "angelito," and leave the past behind.