What does the Flag field mean?

The flag field contains the 802.11 capability information in hexadecimal. 
It's documented in section of the 802.11b spec. 
To save you having to go and look, the bits are:

0001 ESS (Infrastructure)
0002 IBSS (Ad-Hoc)
0004 CF-Pollable
0008 CF-Poll Request
0010 Privacy (WEP)
0020 Short Preamble
0040 PBCC
0080 Channel Agility
FF00 Reserved
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