On Conditional defines page you set symbols that should be pre-set for each parsed unit. Typically, you'll want to set Compiler defines and Project defines.
Symbols
List of pre-set symbols
Compiler defines
If checked (default), symbols CPU386, WIN32, and VER90/VER100/VER120 will be pre-set.
Console application
If checked (default for applications, which have Generate console application option set), symbol CONSOLE will be pre-set.
Project defines
If checked (default), project symbols (specified in Delphi IDE in Project.Options) will be pre-set.
Disable user defines
If checked (default unchecked), additional user-defined symbols (added in this dialog) will be disabled.
Add
Adds user-defined symbol, specified in entry field.
Rename
Renames selected symbol to a name, specified in entry field.
Delete
Deletes selected symbol.
Clear user
Removes all user-defined symbols.
Clear all
Removes all symbols.
Defaults
Resets all settings (except Console application, which is set according to application type) to default values (as set in Preferences dialog).