Encerradas means 'locked up' or 'trapped'. It's a strong, evocative word that immediately brings to mind a sense of confinement and longing.
In the song, the lyrics
Have you ever been so completely smitten that you can't sleep at night? That's exactly where the singer of Los Palmeras' "Quisiera Volver" is! He's absolutely consumed by thoughts of this person, saying, "que será que por las noches ya no puedo ni dormir" [what is it that at night I can no longer even sleep].
He feels totally trapped by this love, telling her, "me encerradas entre tus rejas y ya no notaron salir" [you enclosed me between your bars and they no longer noticed me leave]. He believes she's too young to understand the depth of his feelings, mentioning she's "solo 14 años" [only 14 years old] and needs more time to grasp what's in a heart. Despite this, he's pleading with her, "por favor pequeña mía escuchar escúchame" [please, my little one, listen to me, listen to me], hoping she'll hear him out. It's a really intense and heartfelt plea for her to understand his overwhelming affection.