Desprevenida means 'unprepared' or 'caught off guard'. It's a less common word than its opposite, 'preparada' (prepared), making it more interesting and memorable.
In the song, Salvador Sobral sings, "Estavas desprevenida e por acaso eu também" (You were unprepared and by chance, so was I). This word beautifully captures the unexpected and spontaneous nature of falling in love, suggesting that neither person was looking for it, but it happened anyway.
Salvador Sobral sings about love as something that just happens to you, not something you sign up for. He opens by refusing any credit for the whole business — 'Não fazes favor nenhum a gostar de alguém' ('You're not doing anyone a favor by liking someone') — and shrugs off inventing it too: 'Quem inventou o amor não fui eu' ('I didn't invent love'). What decides it, in his telling, is chance: he and the person he's singing to were both caught off guard, and it was chance that tangled their lives together — 'De nossas vidas a vida fez um acaso também' ('Life made a coincidence of our lives too'). The recording matches the mood — just his voice over a hushed, jazz-leaning arrangement, closer to someone thinking out loud than to a performance.