The settings form allows you to setup your default directories and other options (such as auto-update).

"Main Menu" = specify the path to 'mainmenu.map'; used for the menu listing in-game of maps that can be loaded
"Shared" = specify the path to 'shared.map'; contains resources that are common between multiple multiplayer maps
"SP Shared" = specify the path to 'single_player_shared.map'; contains resources that are common between multiple campaign maps
"Bitmaps" = only used fo Halo 1 and unneeded for Halo 2.
"Maps Folder" = This is the directory that "Open Map" will browse to.
"Clean Maps" = This is the directory that "New Map" will use to create maps from.
"Plugin Folder" = This is the directory that contains your plugins (*.ent)
"Bitmap Folder" = This is the directory that will be used when extracting bitmaps from a map
"Extract Folder" = This is the directory that will be used when extracting meta, models, etc from a map.
"Always use default maps" = When selected will always use the 'External Map Paths'. When deselected, will attempt to load mainmenu.map, shared.map, single_player_shared.map from current map directory and fall back on the default if non-existant.
"Use registry for settings" = When selected all settings will be stored to and read from the registry (shared across all supported versions & copies). When deselected will read settings from 'settings.xml' file in main directory.
"Perform Auto Update" = Select how often to check for newer versions of Entity; default is 'Daily'