When you’re struck by a brilliant idea (which usually happens to me around 1 a.m.), you don't want to slip away as you spend time unlocking your iPad and opening the Notes app.
No problem. If you tap the Apple Pencil on the iPad's lock screen, you can start writing a note right away. Or you can simply tell Siri to "Take a Note" and she'll ask you what you want to say. The biggest benefit of the Notes app is that it links notes across devices using iCloud.
Notes can also be password locked. If you’re using an iPad that supports Touch ID, you can unlock the note with your fingerprint.
(This how-to is based on my experiences and info on Apple's support pages — where the images sometimes come from.)