Indosserai means "you will wear". While this verb is typically used for putting on clothes, this song uses it in a beautiful, metaphorical way.
Il Volo sings, "Indosserai sorrisi, allegria, ma senza magia" (You will wear smiles, cheerfulness, but without magic). This paints a vivid picture of someone putting on a happy face to hide their true feelings after a breakup, suggesting the smiles aren't genuine. It's a poignant and memorable use of an everyday word.
This song is a powerful ballad about a really tough breakup. The singer is completely heartbroken and imagines seeing his ex again, knowing she'll just look at him and say goodbye. He's lost all hope, singing, "A chi mi dice / Che tornerai / Non credo oramai" (To those who tell me / That you will return / I don't believe it anymore).
The most painful part is how he sees her moving on. He describes how, when people tell her he's suffering, she doesn't care at all. Instead, "Tu sorridi voltandoti / Verso lui" (You smile turning / Towards him). He feels like he's lost everything while she's won, believing she's someone who "non riesci a perdere mai" (you never manage to lose). It's a song about feeling completely defeated by a love that has ended.