Merging layers combines multiple layers together. Pixel, vector, mask, adjustment or image layers can be merged into a new merged layer or into the first available pixel layer beneath it in the layer stack.
The entire document can also be flattened to produce a single-layer document.
Once layers have been merged, they become a single layer and their previous contents are no longer separately editable. There are several ways of merging layers.
In the Layers panel:
A new layer is added one step above the selected layer. This layer is a merged copy of all visible layers.
The selected layers merge down into the lowest layer in the selection.
The selected layer merges with the first available pixel layer beneath it.
A flattened version of the visible layers is added to the Clipboard.
The document will then contain a single flattened layer.