Trago literally means 'a drink' or 'a gulp', usually of an alcoholic beverage. It's a common word, but Carin Leon uses it here in a very poetic way.
In the song's title and chorus, "Cuando la vida sea trago, me la tomo en serio", he creates a powerful metaphor. He's saying, 'When life is a drink, I take it seriously', meaning he lives life to the fullest and embraces every moment—good or bad—with intensity.
Carin Leon kicks this song off by painting a very clear picture of himself. He’s the guy who gave his mom gray hairs, “el que le sacó canas a mamá,” and the family’s official “oveja negra” (black sheep). He admits he was trouble from the start, only going to school for the pretty girls and becoming a heartbreaker after one broke his heart. He’s fully aware of his reputation as a “mujeriego” (womanizer) and even offers a half-hearted apology, but quickly follows it up by saying he’s always loved chaos and he’s not going to change.
The whole attitude of the song is summed up in its amazing title line: “cuando la vida sea trago / Me la tomo en serio” (when life is a drink / I take it seriously). He's not here to be a role model; he's here to live his life to the fullest, even if that means being a self-proclaimed “bad boy.” He warns everyone not to try and change him, because he’s happy the way he is. It's a bold, funny, and confident anthem about accepting yourself, flaws and all, and just enjoying the ride.