Plastilina literally translates to "play-doh" or "modeling clay". It's an unexpected and charming word to find in a song about forbidden love.
In the lyrics, the singer says, "Que alguien le explique a mi corazón, pero con plastilina porque es terco" (Someone explain to my heart, but with play-doh because it's stubborn). He uses this playful metaphor to express how incredibly stubborn his heart is, implying it needs a simple, hands-on explanation like one would give a child, because it refuses to understand that he shouldn't love his secret admirer.
Jhonny Rivera brings us a song about a man caught in a really tough spot! He's got a secret love, and his heart is just not cooperating. He sings, "Que alguien le explique a mi corazón, pero con plastilina porque es terco no no" [Someone explain to my heart, but with play-doh because it's stubborn, no no]. He knows he shouldn't love this new person because he's already married, but he can't help wanting to see them and run to them.
It's a classic dilemma: his wife is good, and he never wants her to find out. He's even deleting messages without reading them! But this secret love is driving him crazy. He acknowledges that it's not easy for his secret love to accept only "migajas" [crumbs] of his affection when they give their all. He feels this new love has given his life a "nueva ilusión" [new illusion/hope], and he describes his heart as a "potro sin frenos" [colt without brakes], jumping fences and not caring about the pain because love is an "anestesia" [anesthetic] for the wounds. It's a really honest look at the messy feelings of forbidden love.