From Halopedia, the Halo wiki
Propulsion pods are devices used to give human space vehicles maneuvering capabilities and the ability to enter Slipstream space. They are an integral component of DCS light freighters, which, without a propulsion pod attached, are simply cargo containers lacking any maneuvering systems or a slipspace drive. In this respect, their function is similar to tugboats used to push supertankers out to sea. They connect the rear of the vehicle with a powerful magnetic coupling and are bulbous, around 1/10th the size of a DCS freighter cargo container. Propulsion pods are extremely utilitarian in design; most of their interior components are exposed, apart from the slipspace drive, which is encased in a protective cowling.
Propulsion pod-equipped freighters were the most common type of craft used by the UNSC Department of Commercial Shipping to ferry supplies and agricultural produce to and from the planet Harvest by 2524. They were also used during the escape from Harvest. After the first meeting with the Covenant went sour, Sif, following a plan by Jilan al-Cygni, moved hundreds of propulsion pods from the Tiara out into space, to coordinate weeks and months ahead of Harvest, along its orbital path several days before the Covenant warship, Rapid Conversion arrived.
When the Covenant began attacking Harvest, they fled from surrounding cities to Utgard, the capital of Harvest and the location of the space elevator depot. As war waged around them, +250,000 humans managed to escape the planet by packing into 236 freight containers which where loaded into the space elevators along with two "grease buckets", maintenance containers that carried Johnson's and al-Cygni's militia men. The freight containers that survived, along with the two "grease buckets", then traveled the length of the space elevator strands, passing the Tiara and jettisoned out into space, where they coupled with the propulsion pods, allowing them to enter slipspace and escape from the Covenant.