From Halopedia, the Halo wiki
A seed world is a kind of Forerunner megastructure designed for the purposes of terraforming and assisting with the Ecumene's mass-scale astroengineering. They were used by the Ecumene to deploy to dead, dying and marginal planets to prepare them for use by the Forerunners. Upon deploying to a given target planet, seed worlds were able to perform such tasks as turning dust into living flora and fauna or draining the soil to grow "machinery of death". Seed worlds were generally controlled by an ancilla and built to perform a purpose, with many continuing to do so long after the Forerunners' extinction.
Seed worlds were constructed at the artificial shield world of Genesis, utilising raw materials mined directly from the celestial satellites placed in orbit around the installation. The seed worlds constructed at Genesis served as a key part of the Conservation Measure, by repopulating the Milky Way after the firing of the Halo Array with life forms gathered from Genesis' own surface.