Letargo translates to "lethargy" or "slumber." It's a less common word that evokes a sense of deep inactivity or dormancy.
In the song, the singer uses letargo to describe a period in her life where she was emotionally dormant, "viví como en sombras, dormida, sin sentir la más leve emoción" (I lived as if in shadows, asleep, without feeling the slightest emotion). The word beautifully captures her past state of emotional unresponsiveness before love awakened her.