If you use the probability percentage field on your Opportunities you'll probably want to build reports that include a weighted Opportunity value, ie, the amount of the Opportunity multiplied by the probability.

This is easy to achieve by adding a calculated column to your report, similar to the one below.  (The actual formula might differ from this if your report isn't based on Opportunity records.)

amount * probability /100

If you want to see a total value for the weighted amount at the bottom of the resulting report column, tick the checkbox next to Show a grand total below this column and select Total from the dropdown picklist that appears.

In addition, to make the output figure look correct in your report, open the Advanced tab on the calculated column and select currency from the dropdown picklist next to Display as.

Remember, if the probability field in any of your Opportunities contains either 0 or no value at all, the weighted amount will show as either 0 or with no value at all too.