The full phrase in Italian is "Era la notte, e il cielo avea stelle" which translates to "It was the night, and the sky had stars." However, the line is often shortened to "Era la notte" and is most closely associated with the phrase "It was you."
The aria is sung by the character Riccardo, who is confessing his love for Amelia, despite being engaged to another woman. The aria is a powerful and moving expression of his emotions.