Salto mortal literally translates to "deadly leap" and is also the term for a somersault. It's a dramatic phrase you won't hear in everyday conversation.
In the song, Shakira sings "Dimos un salto mortal" (We took a deadly leap), using it as a powerful metaphor. She's describing taking a huge, potentially disastrous risk in a relationship, a leap of faith into the unknown.
This song feels like the moment the clouds finally part after a long storm. Shakira sings about moving past a difficult time in a relationship, deciding to forgive someone even if she's not sure about their mistake. She's done with pretending things are okay, singing, "No nos sirve más fingir" [Pretending doesn't work for us anymore]. It's a song about taking a huge, scary risk for love, which she calls a "salto mortal" [a mortal leap, or a huge risk].
Throughout the song, there's this powerful sense of hope returning. She decides today is a "día especial" [special day] to believe in a second chance. After all the confusion and tears, she sees a "faro en la oscuridad" [a lighthouse in the darkness] again. It's a beautiful declaration of choosing to move forward and give love another try, no matter what happened yesterday.