Mascherina is the diminutive form of maschera (mask), meaning "little mask". It's an interesting word because it immediately brings to mind the imagery of commedia dell'arte, which is central to the song's theme.
In the context of the song, it refers to the masks worn by characters like Arlecchino and Colombina, symbolizing the facades people wear and the hidden emotions beneath them. It's a unique and evocative word that sets the tone for the entire piece.
Gino Paoli reaches for commedia dell'arte's classic pair, Harlequin and Colombina, to describe a love that only survives behind a mask. He paints himself as fragile and patched-together — "Rappezzato come la mia vita" ("Patched like my life") — and admits, "Sei una marionetta che non sa neppure quel che fa" ("You're a puppet who doesn't even know what he's doing"). Only her smile changes that: "Ma se sorridi, tutto sei per me, e vivo solo in te" ("But if you smile, you are everything to me, and I live only in you").