The square brackets in your example serve no purpose. However, if fieldName was called "field name", then you'd totally need the square brackets, because look what happens:
tblName.field name
tblName.[field name]
In the first, Access thinks tblName has a method field and name is an argument of that method.
In the second, the reference is made correctly. That's why you should call your things without special characters, like space, that way you avoid having to type the annoying square brackets. By the way, Access adds them automatically for you most of the times.
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