Descuidé comes from the verb descuidar, meaning "to neglect" or "to not pay attention to". It's the emotional core of this heartbreaking song of regret.
In the lyrics, the singer confesses, "Tontamente descuidé... de tu amor" (Foolishly I neglected... your love). He uses this single word to express how he took his partner for granted, stopped being thoughtful, and ultimately lost her. It's a powerful admission of fault that sets the tone for the entire song.
This song is a massive confession of regret. The singer had a partner who was absolutely perfect for him. They gave him everything without asking for anything in return: "Tú me entregabas todo sin pedirme nada" (You gave me everything without asking for anything). His life was complete. But then, as he says, "me confié / Y tontamente descuidé" (I got complacent / And foolishly I neglected).
He admits he completely messed up. He stopped being the thoughtful guy he used to be, confessing "ya jamás te mandé flores" (I never sent you flowers again). He became cold, indifferent, and even calls himself "un animal que te ofendía" (an animal that offended you). Eventually, his partner got tired of it and left. Now the love is gone, and he's left with the painful realization that he lost something great, singing "ahora por ti me estoy muriendo" (now I'm dying for you).