(The images are from the iOS app.)
A layer refers to a clear sheet of canvas.
When you use layers, the image can be separated into tiers, not dissimilar to traditional cel animation.
For example, you could split coloring into layers and create a separate one for both skin and hair, making it easier to work with.
▼How to add layers
(The image is from the iOS app.)
Tap the + mark to add layers.
▼How to change a layer name
(The image is from the iOS app.)
Long press the default layer name to change it.