I need some help understanding some formulas that already exist on a workbook
that someone else made. Here is an example of one:

=IF(AND('Past Due Report' !E40, 'Past Due Report' !E4<16), 'Past Due
Report' !G4,0)

'Past Due Report' is the name of sheet 1 in the workbook, and the formulas
exist on sheet 3. What does the exclamation point mean?

I hope I explained my question clearly... thanks to whoever helps me!