Vagabundo literally translates to "vagrant" or "tramp", but in a more colloquial and often affectionate sense, it can mean "wanderer" or "roamer".
In this song, the phrase "Coração Vagabundo" (Wandering Heart) is used to describe a heart that can't settle down, that falls in love easily and repeatedly, and doesn't commit to one person. It captures the essence of a carefree, perhaps even reckless, romantic spirit.
"Coração Vagabundo" ("Vagabond Heart") is about a man who feels he has turned reckless after a broken heart.
He reaches for cachaça to numb the hurt, and admits his heart has no owner and no direction anymore. Drink after drink, he keeps falling in love again, chasing one passion after another instead of settling down.
It captures that restless, can't-sit-still feeling of a heart that wanders from one attraction to the next because it doesn't know how to stay.