Estallar means 'to burst' or 'to explode'. It's a dynamic and intense verb that conveys a strong sense of impending release or breaking point.
In the song, the lyric "A punto de estallar" (On the verge of bursting) vividly describes the singer's emotional state. He's at his limit, filled with adversity, and feels like he's about to erupt with emotion, making this word particularly impactful and memorable.