I believe some drives (mostly older?) have issues reading DVDs which contain less then 1GB of data, so if you burn less then 1GB of data it will pad the disc with additional data to bring up up to 1GB. If your project already has more then 1GB, that option will not do anything.
What is "DVD High Compatibility Mode"?
The DVD High Compatibility Mode is used to aid compatibility with certain DVD drives and players (by padding the disc with dummy data to a minimum of 1GB), but it will greatly increase the burning time. You should enable this option only if you encountered incompatibility issues with some DVD drives/players.