Take a look at the "The difference between relative and absolute
references" topic in the "About cell and range references" topic in XL
Help.
Essentially,
When copying/dragging/filling formulas, putting a $ in front of the
column character(s) preserves that column in the copied cells. Likewise
a $ in front of the row number.
Put these formulae in the indicated cells:
B1: =$A1
C1: =A$1
D1: =$A$1
Now select B1
1, grab the fill handle (lower right corner of D1) and
drag down a few rows. Look at how the formula changes in rows 2 and
greater.
Now put these formulae in the indicated cells:
A2: =$A1
A3: =A$1
A4: =$A$1
Select A2:A4 and drag to the right. See how the formulae change.