"Promesa" is Ñengo Flow doing what he does best: a slow-rolling, very grown-up reggaeton come-on. He opens by telling a girl to let the night take over, "Deja que fluya la ilusión" (let the excitement flow), and from there he is not subtle at all. He calls himself the "bellaco" in the room, which is Puerto Rican slang for someone who is horny and up for it, and bellaquera is the mood itself.
The chorus is the part worth learning. She "quiere aventuras nuevas porque ya se cansa de lo mismo" (wants new adventures because she is tired of the same old thing), and she does not want anyone promising her "amores con sueños perdidos" (love built on lost dreams). So the title's promise is exactly what she is refusing. He also insists "la nena está grande y se defiende solita" (the girl is grown and looks after herself), which is a nice counterweight to all the boasting.
Heads up: the verses get very explicit, and "Real G4 Life" and "Eazy" are just tags, the name of his "volumen 3" series and a shout-out.