Whitespace refers to the amount of white (blank() between objects on a staff and/or spacing between staffs.
MusiCAD will usually calculate the necessary amounts of whitespace automatically. However, there are are a few situations when you might want to interfere:
adjust whitespace between two notes by moving the right one to the right using <alt->
adjust whitespace between two lines by inserting a line-break and 'move' downwards using <alt->
a (deprecated) method of adding whitespace between objects is inserting a whitespace-object using <w>