Mancuernan comes from the verb mancuernar, which means to team up or join forces, often with the connotation of working together closely, like a pair of cufflinks (mancuernas).
In the context of this song, it's used in a romantic sense: "Yo te enseñaré, chamaca, cómo se mancuernan dos" (I will teach you, girl, how two people join together). It's a unique and charming way to talk about two people coming together in a relationship, making it an interesting word choice for learners.
This song tells a classic story of courtship from a confident suitor's point of view. The singer begins with a question to the trees in a ravine, asking why they aren't green. The answer is that they haven't been watered. This imagery sets the stage for the singer's own quest for love, which he feels is a vital, life-giving force.
The repeated line, "Me puse a amar a una mujer / Con la ilusión de amar" (I started to love a woman / With the hope of loving), reveals his main motivation. He spots a woman in a white dress and immediately decides she's the one. He addresses her directly, assuring her that even if she sees him drinking or wandering, his feelings for her are real. The song ends with a bold and rustic promise: he's going to the ravine to "plant rows of rice" and show her how two people become a couple, using the metaphor of farming to describe building a relationship.