In this part, we will focus at regular layers with raster images (pixel data). There are many tools to edit pixel data (Brush, Eraser, adjustments, filters). But we often don't want to edit the whole layer, but just some part of it.
The selection defines a set of pixels, that are "selected" and can be edited by tools. Precisely, it is a "map" over the whole image, which gives each pixel the value between 0 % and 100 %. This value says, how much color of the pixel can be changed, and how much should be preserved. Selections are not bound to layers (as masks), but there is just one selection for the whole document, which is used for every layer that you work with.
For example, Photopea has the Fill operation (Edit - Fill). By default, it will fill the whole layer with a foreground color. Once you have a selection, it will fill only the selected area with that color.
While masks are visualized with the white and the black color (in their thumbnails), selections are visualized right in the main area of the document with the contour, that is drawn between selected and unselected areas. We can say, that operations are applied inside the selection, and not outside of it. Here you can see an example of the selection, and the result, when you draw on top of it with a Brush tool.
Working with Selections
Once you have a selection, there are several basic operations for changing it. The basic operation is inverting the selection - it will select unselected pixels and unselect selected pixels (available in Select - Inverse). You can expand the selection by several pixels (i.e. add pixels close to the selection into the selection), contract it by several pixels (the opposite operation to expanding), or feather - apply a blur to the selection, to make a smooth transition between 0 % areas and 100 % areas. All this is available at Select - Modify - Expand / Contract / Feather.
Selected pixels can be Cut, Copied or Pasted (Edit - Cut / Copy / Paste, or Ctrl + X / C / V). This even works between different documents.
Deselection (removing the selection) is possible through Select - Deselect (or Ctrl + D).