How do I begin to understand the data in the Wifi section of WireShark, please?

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Some ideas:

1. Start with a book - something like Gast's 802.11 Wireless Networks
2. Some good websites exist, too, such as this one.
3. There are some wifi sessions from Sharkfest over the years. Youtube has many of them: https://www.youtube.com/c/SharkFestWiresharkDeveloperandUserConference/videos.
4. The 802.11 specification is there, too, but it is a large document. Recommended only for advanced users.
5. wi-fi.org has some good information, too, if you dig.

This is just a start - there are other books and certification programs that you could study for that provide knowledge. Good luck!