Terra, better known as "Earth", was the original home to the human race in the Wild Arms series. At some point in each game's history, Mankind left Earth and created space colonies, with Filgaia being the farthest (and apparently final) colony.
The name, Filgaia, itself is a pun on this relationship - with Gaia being another name for Earth, and the original form of the name "Fargaia" winds up being "Far Earth".
Wild Arms 3
Terra's civilization was apparently destroyed by the Metal Demons sometime before the events of the game. Zeikfried informs us that the genetically-engineered Demons couldn't stop their blood-lust and ravaged the world until they simply expired or went off to other Terran colonies, such as Filgaia. This would serve to explain why no one from Terra has come calling in all this time.
Wild Arms TV
The "Great War" with the Metal Demons is made more explicit in the tail end of the serires. After the terraforming project on Filgaia began to malfunction, Terran forces went to commit a purge of the planet, engulfing Filgaia in a great civil war. Filgaia was abandoned, but it seems that Terra didn't escape the consequences of the terraforming disasters either, since it's never heard from again.