Malherido translates to "sorely wounded" or "badly hurt." It's a more intense and dramatic way to express pain than simply saying "herido" (wounded).
In this song, Christian Nodal uses malherido to convey the deep emotional pain he feels without his lover's touch, singing "Sin tus caricias yo me siento malherido" (Without your caresses, I feel sorely wounded). This word elevates the emotional stakes and makes his longing more palpable.
Christian Nodal opens by admitting he forgot the one thing that mattered: "Se me olvidaba decirte que te amo" [I forgot to tell you that I love you]. The rest of the song is him making up for it, line by line, no half-measures, no games.
He lays out exactly what she does to him: without her kisses he is wounded, without her he does not know how to live. Then the chorus turns it into a trade, not a plea: "Te quiero y me quieres" [I love you and you love me], his body, his life, his soul, all hers. He ends the same way every time: "Mi vida yo no entendia" [my life, I did not understand], "Hasta que te conoci" [until I met you]. Happiness, peace, none of it made sense before she showed up. It is a ranchera built on mariachi horns and one simple idea: he was missing something and did not know it until she filled it in.