Fidelidad means "fidelity" or "faithfulness," especially in a romantic relationship. In the song, the singer insists on "toda mi fidelidad" to reassure his partner that he is loyal and has not betrayed her. It is a meaningful, slightly formal word that reveals the song’s central conflict.
Makano sings to the woman he is with now, who is upset because his ex keeps calling him. He insists it isn't his fault that she still hasn't forgotten him, and asks his partner to wait, listen and believe him: he is a one-woman man.
The chorus is the plain confession of the title, "Te amo" (I love you), and he tells her that being with her is what makes him happiest. In the bridge he begs her not to let their love burn out. The last verse returns to the opening problem, the phone that keeps ringing, so the song ends on his promise rather than on a resolution.