Hub Fleets are fleets that fulfill building queues. For instance, if you want to build another Cryolab on Vasilis but are short of circuits, you can queue the building and hub fleets from other planets will automatically go into action to get the needed parts to Vasilis.
Building queues are unlocked after you conquer your second planet (Vasilis)
Hub fleets are unlocked after conquering the fourth planet (Orpheus).
When a hub fleet sends out parts, it sends out only as many ships as needed. If you have 10 ships in Ishtar Gate's hub fleet and you only need to send enough parts to fill one ship, it will only send one ship. This lets one planet feed many others at once.
However, this is currently buggy, and the fleets will prioritize the queues over their home planets' needs. This can leave a planet short of what it needs, especially if you keep the majority of your goods on a hub planet (a common strategy as it makes autoroutes much more efficient).
Instead, it's recommended that you designate a hub planet, store much of your excess there, and have all hub ships attached to that planet.
To add ships to a hub fleet, take them to the planet you want them to be attached to. Once there, highlight them in the Fleet screen and click "Join Hub Fleet" on the right.
To remove ships from a hub fleet, go to your Fleet screen, click Hub Fleets, find the one you want, and divide them out. If you have ten ships and you want to remove all ten, simply divide ten out.
While enlisted in a hub fleet, ships cannot be renamed or controlled.