Dock Springs are found in Freeports. They prevent your Spaceship being pushed around by other Spaceships. This can help prevent theft when ejecting items out of your 
Cargo Ejector.
You can easily dock your Spaceship by landing(touch) one of these Dock Springs, and easily undock by flying away from it. Once docked, your Spaceship will have a slight pull towards the Dock Springs. It's noticeable. They also disable your thrusters saving fuel.
Ship embiggeners and Ship Shrinkinator cannot be used when ship is docked.