If you want to know how box-shadow works, this is the explanation:
using box-shadow property you can add more than one shadow to an element. Comma (,) is used to declare multiple shadow properties in one line.
in the given challenge you are assigning two shadows to box-shadow property
the first one has 0 pixels horizontal radius, 10 pixels vertical radius, 20 pixels spread, black color (0,0,0) with hue 0.19
the second one has 0 pixels horizontal radius, 6 pixels vertical radius, 6 pixels spread, and black color (0,0,0) with hue 0.23
By assigning the above shadows, you are creating a dark shadow closer to the box (second with 6 pixels spread and hue 0.23) and dull shadow further from box (the first one with 20 pixels spread and hue 0.19)
Follow the below link if you need further more explanation and syntax of box-shadow: CSS box-shadow property
It’s good to struggle a bit, but not for too long. So don’t be afraid to ask for help when you need it. Sometimes all we need is just a fresh pair of eyes to guide us.
I would also highly recommend using something like MDN for documentation.