To the right of the bang symbol is a cell reference - F22 in this case - and to its left is the name of the worksheet concerned, which is worksheet 'weighted ave mu' in your workbook in this case.
So, 1 + 'weighted ave mu'!F22 should be read as
'in the current cell add 1 to the value of cell F22 on the worksheet named 'weighted ave mu' in this workbook.'
Excel does not attach meaning to the worksheet name - that was set by whoever named it 'weighted ave mu', not by Excel, so I would not read too much into the mu part - it might be markup, it might not, but Excel is not going to mind whatever it means.
-Stewart