Infierno literally means "hell." While it's a very strong word, it's often used in Spanish songs and poetry as a powerful metaphor for emotional suffering.
In this song, Enrique Iglesias expresses his desperation by asking how he can "escaparme de este infierno" (escape from this hell). This dramatic use of the word vividly paints a picture of his torment and longing, making it a very memorable line.
Have you ever been so completely in love with someone that you can't even say the words? That's the whole story of this song. Enrique Iglesias is totally obsessed with someone, telling us, "No hay un minuto al día que no piense en ti" (There isn't a minute of the day I don't think about you). He has "tres palabras" (three words) he's desperate to say, but every time he gets close, the words "se pierden al mirarte" (get lost when I look at you).
This feeling is so intense he calls it an "infierno" (hell) that he needs to escape. His heart "no quiere callar" (doesn't want to be quiet) because he feels that love can be fleeting, that it "llega de pronto y huye sin decir adiós" (arrives suddenly and flees without saying goodbye). He decides he won't let the chance slip away, promising to follow this person until he finds them and finally says the words he's never said before.