Literally meaning "bulletproof", antibalas is a cool compound word formed by combining anti (anti-) and balas (bullets).
In "De Una Vez," Selena Gomez uses this word as a powerful metaphor for emotional resilience. She sings, "ahora este pecho es antibalas" ("now this chest is bulletproof"), symbolizing that she has become so strong after a heartbreak that she can no longer be emotionally wounded. It's a memorable and empowering image that captures the song's core message of healing.