Aventar literally means 'to throw', but in this song, it's used as a popular slang phrase meaning 'to go for it' or 'to take the plunge'.
The singer is hesitating to approach a girl at a party, but then decides, "mejor me voy a aventar" (I'd better just go for it). This phrase perfectly captures that exciting, nerve-wracking moment of taking a social risk, which kicks off the whole story of the song.
Junior H kicks this track off right in the middle of a late night party, surrounded by drinks, friends, and what he calls Barbie' bien preciosas [gorgeous Barbies] on the dance floor. He is drinking cold beer and his buddy passes him un tabaco pa' embaisar [a cigarette to light up]. Then he spots a girl with lips and a face so striking that his chest starts acting up: El corazón siento me empieza a palpitar [I feel my heart starting to race]. He literally jokes that he has no clue what that feeling even is, guessing it must be what people call butterflies (creo, dicen mariposas). Even though he is nervous to say hi, he takes a chance (mejor me voy a aventar [I'd better go for it]), taps her shoulder, and she pulls him straight onto the floor.
They dance song after song with her body pressed right against his, and while he admits he is terrified of catching feelings (Tengo yo miedo y no me quiero enamorar [I'm scared and I don't want to fall in love]), it is already too late. Junior skips the details with a smirk: Ya ni les cuento, ya saben lo qué pasó [I won't even tell you, you already know what happened]. She ends up in his bed until sunrise, but the high does not last. The morning comes, she vanishes, and now he is stuck right back at the club (el antro), smoking (fumando un gallo) and scanning the crowd hoping to run into her again. By the end, he is literally begging the moon to pass along a message (Luna, dile tú... [Moon, you tell her...]) to bring her back.