explain a=fill(x,y,'k') set(a,'facealpha',0.2) in matlab

The code a=fill(x,y,'k') creates a filled polygon bounded by the vectors x and y with a black color. The output variable a is a handle to the graphics object created by the fill command.

The code set(a,'facealpha',0.2) uses the graphics object handle a to set the transparency of the filled polygon to 0.2. A value of 0 represents completely transparent, while a value of 1 represents completely opaque.