Fugaz translates to "fleeting" or "ephemeral." It's a beautiful and somewhat poetic word that isn't as common as everyday adjectives, making it stand out.
In the song, the artist uses it to describe a brief, almost dreamlike encounter: "Tan fugaz que ayer la vi y hoy no sé nada de ella" (So fleeting that I saw her yesterday and today I know nothing about her). This word perfectly captures the transient nature of the moment and the lingering sense of loss, making it memorable and evocative.
"Una Rosa" is about holding on to love you never got to fully give. The singer still carries a rose and an old letter meant for someone he lost touch with, wishing he'd said what he never managed to say.
Years later he still thinks of her, calling her the prettiest one in the room, and admits he's still the same fool behind the scenes, wondering if she'll ever love someone else the way he loved her. He's trying to move on and be happy with someone new, but everything he learned about love, he learned loving her, and he tattoos those memories into who he's become.
By the end he's not asking her to come back, just hoping that if she ever hears this song, she'll remember that she was the best part of his story.