Diera is the imperfect subjunctive form of the verb dar (to give). While dar is common, this specific subjunctive form is less frequent and adds a layer of hypothetical desire or regret.
In the song, Vicente Fernández sings, "Yo qué diera por no recordarte" (What would I give to not remember you) and "Yo qué diera por no ser de ti" (What would I give to not be yours). This expresses a deep longing and a hypothetical wish to undo his feelings, making diera a poignant and engaging word choice.
Vicente Fernández's "Corazón Corazón" is a passionate plea from someone who can't imagine life without their love. He admits, "Es inútil dejar de quererte / Ya no puedo vivir sin tu amor" [It's useless to stop loving you / I can no longer live without your love]. He's practically begging, "No me quieras matar, corazón" [Don't try to kill me, heart].
It's a dramatic song where he reminds his beloved of their shared past, saying, "Recuerda el camino donde te encontré" [Remember the path where I found you] and "Recuerda un poquito quién te hizo mujer" [Remember a little who made you a woman]. But if she still decides to leave, he makes a heartbreaking declaration: "Te diré con el alma en la mano / Que puedes quedarte porque yo me voy" [I will tell you with my soul in my hand / That you can stay because I am leaving]. Talk about a mic drop!