Reverberar means to "reverberate" or "resound." It's a less common, more evocative word than simply saying something will "echo" or "spread."
In the context of the song, it's used to describe how good things and positive energy will not just be felt, but will powerfully spread and resonate, amplifying the feeling of
"Alegria" is Melim and Natiruts joining two of Brazil's warmest genres, pop and reggae, around one simple idea: joy is a choice you can practice.
The verses lean on the new moon as a small ritual for belief and renewal: if you trust it, you have to go after what you want, but gently, taking it easy and mentalizing rather than forcing it. The line about time healing those who have suffered too much frames the song as encouragement rather than denial of hard times.
The chorus is the payoff: let joy invade and overflow your heart with good energy, and let it light you up. The repeated, almost chanted 'alegria' turns the word itself into the mantra, echoing the reggae tradition of using rhythm and repetition to lift a mood rather than just describe one.