It's an old trilogy which started similar to the early Civilization games. Except instead of cities, you had stars and the space between. Combat was a separate, interesting minigame where the ships moved on a board and fired. Ship customization was a primary mechanic from first game. The second was probably the best.
That's the very short explanation of the mechanics. To explain the name, it's a story about the Milky Way galaxy and the planet Orion. Orion was the capital of the ancients, the ones who seeded the Milky Way and uplifted the many species. But the Antarans sought to destroy the Ancients, and nearly succeeded.
In each game, Orion is an incredibly important star system which holds secret technology of the ancients.
The first game is a typical RTS where Orion is a special, desirable star system. I don't think the events are canon.
In the second game, the Antarans are escaping their dimensional prison. It's a typical RTS except that these invaders periodically break through and try to murder a random planet. One victory option is to discover dimensional travel and conquer Antares, ending the Ancients' war. Another option is to be voted as leader of the Ancient-spawned races, and face the threat as one.
The third game is mechanically a mess, but the setting is quite amazing. The Antarans won, and have presided over the galaxy for centuries. They've genetweaked the ancients' races with great predjudice, reveling in their victory over their ancient enemy's offspring. But then a mysterious extragalactic threat destroys their main army. The races of the Milky Way must take this opportunity to grow, and rediscover the ancient technologies long prohibited to them. The Antarans are weakened, but still vastly superior to their erstwhile slaves. So the races have to tread carefully, and avoid angering the Antarans more than anyone else... Until the time is ripe to strike back.