Sobrio means "sober" in English. It's a key word in this song, as the singer repeatedly contrasts his actions and feelings when he's drunk versus when he's sober.
He uses it to explain that he can only express his true feelings and apologize when he's had a few drinks, implying that his inhibitions prevent him from doing so when he's sobrio. This contrast is central to the song's narrative, making it a very interesting and memorable word.
Have you ever called an ex when you've had a bit too much to drink? Well, that's exactly what's happening in El Arrebato's "Sobrio"! Our singer is calling his former love, admitting that he can only truly speak his mind when he's had a few. He says, "sabes que me cuesta decir lo que siento, pero el borracho no miente" [you know it's hard for me to say what I feel, but a drunk man doesn't lie].
He's full of regrets, calling to apologize and to see how she's doing, wondering if she still loves him. He confesses, "solo no me das" [you don't give me anything when I'm sober], which is why he's drinking and calling. He even says he's drinking "supuestamente para olvidarte" [supposedly to forget you], but it just makes him miss her more. It's a really honest, almost painfully relatable look at how alcohol can lower our inhibitions and make us say things we might not otherwise, all in the name of a lost love.