What is box shabow??

Answer 1

https://www.w3schools.com/cssref/css3_pr_box-shadow.asp

Answer 2

CSS box-shadow properties allows you to create single or multiple, inner or outer drop-shadows. The box-shadow property requires you to set the horizontal & vertical offsets and then you can set optional blur and colour. You can create outset (default) as well as inset shadows.

http://www.corelangs.com/css/box/shadow.html