Usually that is used instead of the VALUE function. It's a trick that takes
advantage of how Excel interprets text (or workaround depending on how you
think of it). It also can be used for boolean values (true or false). All
of the following result in the same value of 1.

=1
=--"1"
=--TRUE
=VALUE("1")

In your formula you are using the text function MID to extract the middle
number from a string of charectors. This results in a text string that must
then be converted to a number.

"Brad" wrote:

There are many formulas that I see that have a "--". What are they used for,
or how do I ask Excel for help to read more about this topic.

Example

=IF(ISERR(--(A1)),LEFT(A1,MATCH(FALSE,ISNUMBER(--MID(A1,ROW(INDIRECT("1:"&LE
N(A1))),1)),0)-1),A1)