The program will create a STL-file using unspecified units, which usually will be set in the slicer as mm.|
Length is the length of the base.
Width is the width of the base. Length and width can be swapped.
Height is, unsurprisingly, the heigth of the base.
Skew is the value that the upper corners will be set back from the lower corner, for beveled edges of the base - leave at 0 if you want them at 90°
Depth is the depth of the depression in the center of the base - leave at 0 if you want none.
Frame (or border width) is the distance from the upper corner that remains at height before the depression starts - it must be >0 even if you want none.