Enganar means 'to deceive' or 'to fool'. It's an interesting verb because it can refer to deceiving oneself as well as others.
In the context of the song, the lyrics state, "Você se engana / Pensando que tudo vai melhorar" (You deceive yourself / Thinking that everything will get better). This highlights a common human tendency to delude oneself in difficult situations, making it a relatable and thought-provoking word.
The whole song takes place in one apartment on a rainy night. He thinks about calling and doesn't — "Não quis ficar aqui sozinho / Pensei em te telefonar" ("I didn't want to stay here alone / I thought about calling you") — and after that the song is just the room reporting back. He keeps mentioning the temperature: "Faz frio nesse apartamento" ("It's cold in this apartment"), and later "Faz frio nessa cama quente" ("It's cold in this warm bed"), which is the line the rest of it rests on — the bed is warm and he is cold anyway.
The refrain is two words. "Não dá" sits somewhere between "it can't be done" and "it's no use" — Portuguese uses it when a plan simply will not work, and here it is being said about a life: "Baby, sem você não dá" ("Baby, without you it's no use"). The bridge is as close as the song gets to explaining itself: "A gente pensa que não tem nada a perder / Aí se lembra / Que amar faz parte de viver" ("We think we have nothing to lose / Then you remember / That loving is part of living"). And the image that holds it together is the window: "Lá fora é chuva na janela / Cá dentro é chuva no olhar" ("Outside it's rain on the window / Inside it's rain in the eyes") — the same weather on both sides of the glass.