Tremò is the past tense of the Italian verb tremare, meaning "to tremble" or "to shake". It is a vivid, old fashioned sounding word that captures a strong, almost physical emotional reaction.
In the line "Ed il cuore mi tremò" ("And my heart trembled"), the singer describes his heart shaking with nervous excitement at the sight of someone he loves, a classic romantic image from this era of Italian pop.
This is a classic 'see you later' song, but with a twist! Adriano Celentano mixes Italian and English to tell a simple story of love and temporary separation. He remembers the exact moment he fell for his love, Mary Lou: "Un giorno mi guardasti tu / Ed il cuore mi tremò" (One day you looked at me / And my heart trembled). He's completely devoted, promising she's his "unica passione" (only passion).
Now, he has to leave, but he makes it clear it's not forever. He promises, "Tornerò, Mary Lou" (I will return, Mary Lou). The best part is how he plans to come back. He says his horse knows the way home and will bring him back "come il vento" (like the wind). It's a sweet, dramatic, and very romantic declaration of love from a man on the move.