Colpo di spugna literally translates to "sponge wipe" or "stroke of a sponge". It's an idiomatic expression meaning to wipe the slate clean, to make a clean sweep, or to forget about past mistakes and start anew.
In the context of the song, it suggests a desire to erase the awkward or stagnant moment the couple is experiencing and move on to something fresh. It's an interesting phrase that captures the feeling of wanting a fresh start in a relationship.
"Un minuto fa" ("A minute ago") is Ligabue's snapshot of how fast an ordinary afternoon can turn. Two people are together, bored, more distracted by a phone screen than by each other. The song keeps circling back to the same idea: a minute ago things were one way, a minute from now they might be something else entirely.
The narrator notices the small details of a stalled relationship: a fly that won't leave, a phone's glow lighting up someone's face, the urge to open a window just to change the air in the room. He wonders whether to go out or stay in, whether someone's coming for dinner, whether it's worth trying to fix things fast or just letting the afternoon run its course.
By the end, the chorus turns almost resigned: it only takes a minute for the world to change, and once it starts, it goes all the way. It's a song about how little it takes for a quiet moment between two people to slip into something else.