Hi Edwardbattistini,
Welcome to Affinity Forums :)
You're not the first requesting this. I've added this thread/request to the Common Features Requests index, to keep all related info/user comments in one thread.
It has nothing to do with your display. It affects the number of pixels in your document - effectively doubles the number of pixels in your document while still maintaining a logical document size. For example a 800x600 pixel document, if Retina is ticked will have 1600x1200 real pixels, but the UI will show you everything in terms of 1x pixels (as if you were editing a conventional 800x600 document). That way you can work on a standard resolution document, while adding higher resolution detail for Retina output.
When creating pixel layers, they will be at the real resolution, so when you preview a retina document at 2x you will see 1:1 pixels, but if you view at 1x each pixel you see is effectively resampled from a 2x2 pixel group.