Sballa is a colloquial and energetic verb that means "to blow your mind," "to get you high," or "to thrill you immensely." It's often associated with a strong, exhilarating feeling, sometimes even with the effect of drugs, but in a broader sense, it refers to something incredibly exciting or overwhelming.
In the song, Lorenzo Fragola uses it to describe the profound impact of the "light" (luce) he's singing about: "Sei luce che arriva e che sballa" (You are light that arrives and blows your mind). It conveys an intense, almost intoxicating effect that this person or feeling has on him, making it a very engaging and memorable word.
"Luce che entra" (Light That Enters) is about someone whose presence heals old wounds. The opening verse counts up everything a past love gave and took: steps taken, kisses, a mistake. It settles on a simple idea, stated outright in the first chorus: every wound leaves an opening, and light gets in through exactly that crack.
The chorus repeats "Sei luce che entra" (You are light that enters) next to "Sei luce che arriva e che brilla" (You are light that arrives and shines). Fragola isn't describing a flawless love. He's describing someone who shows up anyway, warms things, and makes it easier to let go of what already happened.
By the bridge he stops trying to explain it: "Non mi chiedere cos'è, che ci unisce, perché non ce n'è" (Don't ask me what it is that unites us, because there isn't one). There's no rational reason for the connection, and he's made peace with that.