Mossissimo is an intensified form of mosso, meaning 'choppy' or 'rough', typically used to describe the sea. The suffix -issimo is used in Italian to express a superlative degree, making it 'very choppy' or 'extremely rough'.
In the song, Ariete sings about a beautiful memory "Tra le onde di un mare mossissimo" (Among the waves of a very choppy sea). This word paints a vivid picture of a turbulent yet memorable experience, suggesting that even in difficult or chaotic situations, beauty and understanding can be found.
The song opens on a room that has shrunk: "Ma quant'è piccola 'sta casa, me la ricordavo più grande" (But how small this house is, I remembered it bigger). ARIETE's answer is that maybe the house is fine and she was the one who was small, always barefoot. Walking barefoot comes back twice and means opposite things: first the fear that "nessuno si accorga di me" (nobody will notice me), then the sand on the shore doing exactly that. Between them sits the line the whole friendship rests on, "Le bambine che eravamo sono grandi così in fretta" (The little girls we were grew up so fast).
The chorus asks for something oddly specific. Sugar first thing in the morning, and a guarantee: "Che non siamo solo gocce sopra le finestre" (That we're not only drops on the windows), "Binari che si incrociano in galleria" (Tracks that cross each other in a tunnel). Both images are about things that meet and then don't. Angelina Mango's verse answers with a better one, a kid in a rough sea who "Galleggiavo come un tappo di sughero" (I was floating like a cork) and worked out from there that even the worst day blows off with the wind. The advice she lands on is unglamorous and probably right: "Basta prenderla così com'è" (You just have to take it as it is).