Aviation 'Night' has several context with different times (and each authority has their own way of specifying it). However, in a nut shell night is important for
1 - when must you have a night qualification/rating (generally civil twilight to civil twilight - roughly 30-45 minutes before/past sunrise/sunset - and specifically 30 minutes in the UK)
2 - when does a take off or landing count for night currency (generally before or after nautical twilight - or about an hour before/after sunrise/sunset )
3 - when must navigation lights be displayed (before civil twilight, 30 after sunset/30 before sunrise in the UK)