Complacerla means "to please her" or "to satisfy her". It's a verb that goes beyond simple happiness, implying a deep desire to cater to someone's wishes and make them content.
In the song, Christian Nodal sings, "Me gastaría mi tiempo solo en complacerla" (I would spend my time just pleasing her). This line beautifully expresses a profound devotion and commitment, highlighting the singer's willingness to dedicate himself entirely to his partner's happiness, making it a very enticing and romantic word.
The whole song is one man telling another to stop running his mouth about his own wife. Nodal opens cold, no greeting: "A mí no me diga, a mí no me cuente" (Don't tell me, don't tell me about it). Then he draws the line outright: "Pero si no va a hablar bonito" (But if you're not going to speak nicely), "Mejor no hable" (better not speak at all). He keeps the formal usted the whole way through, which stings more than shouting would. The best moment is the one about her back: "Amigo, la espalda de una dama" (Friend, a lady's back) "Es pa' llenársela de besos toda entera" (is for covering completely in kisses), and God put it there last for a reason, definitely not so a man like you could gossip over it.
Then he switches to what he would do with her instead. "Si yo tuviera una mujer" (If I had a woman) "Como la suya, y se lo digo con respeto" (like yours, and I say this with respect) sets up the chorus, and that respect is doing a lot of work, because what follows is "Me encargaría de quererla al 100%" (I would make it my job to love her one hundred percent) and "Le juro que le besaría hasta la consciencia" (I swear I would kiss her to her very soul). He finishes on a warning rather than a threat: "Pues mientras que usted está aquí hablando de ella" (Because while you are standing here talking about her), "Alguien se sueña a su lado" (someone is dreaming of being by her side). He never says who.