Can anyone explain the following formula to me:

=Data!$A$5:INDEX(Data!$A:$A,MATCH(9.99999E+307,Dat a!$A:$A))

I am setting up a dynamic chart and found help on a website, I have used the
formula (displayed below) as a dynamic range formula for AllDates that
expands when more data is added to the column. The insructions stated that in
the Define Names dialog box, type AllDates in the Name Box, and enter this
formula in the Refers To box:

=Data!$A$5:INDEX(Data!$A:$A,MATCH(9.99999E+307,Dat a!$A:$A))

The formula does work but I don't understand it, can anyone explain the
formula to me?
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Many Thanks
Cath