They are there to format the position of the card. It so you can imagine what the card placement would look like on a table. Like a picture, but with text.
So imagine a diamond shaped spread.
So instead of
1
23
4
You do
**1**
2***3
**4**
To get that diamond shape in text form.
Edit:
The asterisks don't mean anything in and of themselves. You can use any symbol you want:
--1--
2---3
--4--
or
..1..
2...3
..4..
or
==1==
2===3
==4==