It means that the column and row references are locked.
for example, if you typed =SUM(A1,A2) in cell A3, cell A3 would contai
the sum of values in A1 and A2. If you then copied cell A3 and paste
into cell B3, excel would convert the formula to =SUM(B1,B2).
If instead you typed =SUM($A$1,$A$2), when you pasted the formula th
cell refences would remain the same as they had been locked.
You can also lock either column or row.
Also, highlighting the a cell reference in the formula bar and hittin
F4 will lock the cell refernce for you so you don't have to type th
dollar signs. Hitting F4 repeatedly will toggle through the lockin
choice