The simple way to understand that combination is that pack converts "stuff" to an octet string and unpack converts an octet string to "stuff" according to template strings. Unpacking with a different template than was used for packing reinterprets the octet string, essentially "type-punning" the data. (Type-punning is a C trick where the element of a C union object that is read is not the element that was most recently written.)
This explanation is how I remember when to use pack and when to use unpack.
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