Propulsion Pods are bulbous, around 1/10th the size of a Human Freighter looking somewhat like tugboats used to push supertankers out to sea. They connect the rear of the craft with a powerful magnetic coupling and are used to give those Human space (mainly Freighters) that did not have a Shaw-Fujikawa Slipspace Drive built into them for the ability to enter Slipspace.
They were used during the escape from the planet 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 Elevator 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.