Soso literally translates to 'bland' or 'tasteless'. While you can use it to describe unseasoned food, it's also a perfect word to call a person or situation 'dull'.
In the lyrics, the singer feels he was once "el más soso grano de sal" (the blandest grain of salt)—a powerful metaphor for feeling insignificant and without purpose. Learning this word unlocks the core emotional contrast of the song, where love transforms him into part of "la torre más alta de la ciudad" (the tallest tower in the city).
This song is all about feeling small and then being lifted up by someone's love. The singer sees himself as totally insignificant, calling himself "la última gota del mar" (the last drop in the sea), "una simple miga de pan" (a simple breadcrumb), and even "el más soso grano de sal" (the blandest grain of salt). He admits he can be difficult, saying "Sé que estoy algo insoportable" (I know I'm a bit unbearable).
But then there's this other person who sees everything differently, in full color. Their love is so powerful that it transforms him. Together, they become "la torre más alta de la ciudad" (the tallest tower in the city). The whole song is basically him asking in wonder, "¿Cómo lo haces?" (How do you do it?). It's a beautiful look at how the right person can completely change the way you see yourself.