[project takes so long to print]

I am new to  the world of 3d printing, and to everything surrounding it especially cura.

I made a 3d model of a phonecase and uploaded it to cura, and learned the basics of how to prepare and slice it.

when i sliced, the timer said it would take about 15 hours to print, and a lot of materials and i didnt quite understood why

the thing that takes most of the time of the print is the ''skin'', and I cant figure out anywhere what thats mean.

can someone please explain to me what does skin mean in ''time estimation''?

I am using cura 5.3.0

poco x3 phonecase.3mfpoco x3 phonecase.3mf