Layer
One editable element within a composition that can be manipulated independently of the others.
In practice
A layer is one element, or group of elements, stacked in a composition so it can be moved, edited, hidden or styled without disturbing the rest. Layers also set stacking order: whatever sits on top covers what is below. In a social media graphic, the background photo, a darkening overlay, the headline and the logo each sit on their own layer, so the headline can change without touching the photo.
Often confused with Plane
A layer is a software object inside a file; a plane is a flat surface the viewer perceives in the composition, which may be built from several layers.
Try this
Rebuild a simple poster with every element on its own named layer, then hide them one at a time to see the stacking.