Binary format file containing a database of the physical location of IP addresses. It is used by the GeoIP extension of php. For more information on that, see the PHP Manual - Geo IP Location.
Binary, and possibly proprietary.
The location from which to download GeoIP.dat would appear to be hard-coded into Preferences.cpp:
$ grep -n "GeoIP.dat" Preferences.cpp 1254: s_MiscList.push_back( new Cfg_Str( wxT("/eMule/GeoLiteCountryUpdateUrl"), s_GeoIPUpdateUrl, wxT("http://geolite.maxmind.com/download/geoip/database/GeoLiteCountry/GeoIP.dat.gz") ) );