From Starfleet Commander
Debris is left behind when ships are destroyed in combat. Whenever a ship is destroyed in battle, 30% of the ore and crystal used to construct the destroyed ship(s) remain floating in orbit around the planet as debris. Any hydrogen used to construct the ship(s) destroyed in the battle will be lost, and will not contribute to the debris.
Many attacks are intended solely for debris creation and subsequent harvesting by the attacker, as the resulting debris field from a battle can often contain more resources than the planet had available to the attacker as plunder.
When sending ships on Harvest missions, only Dionysus Class or, in Starfleet Commander Nova, Zagreus Class ships can hold resources from debris fields. Additional ships sent will not be able to hold debris.
Debris can also result in the creation of a moon:
A moon is a celestial body formed from the debris of a large fleet that has been destroyed. For every 150,000 debris left behind after a battle, there is a 1% chance that a moon will be created. The maximum chance that a moon can be created is 20% (3,000,000 debris). The moon will not consume the debris, i.e. the moon will form, but the debris is still there to be harvested. Collecting the debris has no effect on moon formation, as any moon will be created immediately after the battle, and previous debris still in orbit does not increase the chance created by a new battle.