This option will display a window showing a thumbnail for each layer.
Delete Selected - One or more selected layers (using Shift or Ctrl and the left mouse button) will be deleted. The deleted layers are added to the Undo queue.
Duplicate Selected - Each layer selected is individually copied as a new layer. Each new duplicated layer us offset about 5 pixels so they are more visible.
Merge Selected - The selected layer(s) are merged into a new single layer. The old layers are added to the Undo queue.
Reload - All the layers are reloaded in the thumbnails display.
Cancel - the window is closed with no action.
Note that when a project is reloaded or when the system is restarted, all the layers are renumbered to clean up any deleted layers.