The simple but effective lyrics lists things that the singer wants to compare her lover to. You are like a promise, fire in my heart, like a poem, a fresh rain, a guitar in the night and so on. With a beautiful melody by Juan Carlos Calderón no more is needed. The song became an instant classic, which went to be one of the rare Eurosong (or a song sung in spanish) to ever reach the top 10 of the American singles chart.
In the contest the song had many problems. The audio mix of the 1973 contest was horrible and the beautiful string arrangement was completely left out of the mix. We don't know if it was the bad sound or the wrecked nerves, but the lead singer Amaya Uranga had real problems keeping her act together especially with the higher notes. The singing group Mocedades (still active though with several line up changes during its 44 year career) did their best to support poor Uranga, but the overall performance was severly affected.
Even with these difficulties the result was still the second place (among 17 participants) with only four points less than the winner, so we can only guess what the result would have been like if the sound had been what it should and the soloist had kept her cool.
To hear the song in it is full glory I'd suggest you to go to Spotify to hear the recorded version of the song. That way you can hear the song as it is, one of the greatest classics ever to come from the Eurovision Song Contest.
My points 5/5.