The Light Army was an army of light creatures that were trapped inside the Light Prism. They obey whoever has the power of the Prism.
The army was sealed inside the Light Prism sometime before Garnet, Pearl, and Amethyst recovered it in Attack the Light. They allowed Steven to grab it for a moment, unleashing the first creature of the army, White Light, whose death released the other notable members of the army.
After most of the light bosses are back in the prism and the only one left is White Light. Steven and the Crystal Gems discover that the army thought they were being tested, and with some words from Steven, the army turned into light and disappeared peacefully.