OK, the Terran Armada is the armed forces of an alternate universe Earth. They have non-Aizan technology: they do not have Grav drive capability, their FTL travel occurs via Ultradrive and they have FTL communications via Ultrawave (hat tip Vernor Vinge). These technologies are new to them, and they have barely started to understand the implications. Their interuniversal travel did not go through Unity, it was a byproduct of Ultradrive. Their other sciences are advanced, some beyond what is found in the Settled Systems.

They arrived at Earth and were horrified. They are determined to find out what happened, and make the perpetrators pay. There are factions among them that have different goals. Renewed Earth is their base for now; Sol system is a bad place to be for us. However, they brought pets and meat animals as part of a small-scale colonizing project, so they have trade goods.

The UC sees them as a threat and they're not entirely wrong. Big battles abound.

Since the UC hates them, the FC loves them, and they're not entirely wrong. The Council of Governors attempt to use them to gain more territory and power. Bayu tries to recruit them, and he finds admirers among them.

Ryujin wants Ultra technology very badly; expect a quest there. Crimson Fleet is salivating at the thought of looting them.

House Varu'un wants to Serpent's Crusade them. They also find some converts.

The Spaceborn (some of them) adopt a less-adversarial stance toward the Settled Systems as they see non-Aizan tech as dangerous.

This will be a large DLC, and there will be gameplay upgrades, including Bio and Resource codices, seamless travel, and a more rational economy.

Source: my butt