It's worth remembering that Korean Air is obviously a Korean carrier, and thus this text is likely translated from Korean, or at least written by someone who is not a native English speaker.
With that in mind, the answer to your question is likely in the previous sentence, which states "Please check the detailed baggage regulations on the Korean Air website".
In the context of this sentence, "Unregulated" almost certainly refers to baggage that is outside of the regulations.
This does not match the typical english definition of this word, but again, this is likely translated or written by a non-native English speaker, so it's easy to see how they would use the "un" prefix to mean outside of the regulations.
So with this definition, "Unregulated baggage" could be anything from having more than the allowed number of items, to oversized items, or even restricted items or any number of other things - all of which "may not be transported or may be charged an extra fee" as per the statement.