At the time of To Sleep in a Sea of Stars, humans had not yet found a way to directly manipulate gravity (although the possibility was theorized by physicists). Instead, they created pseudo-gravity by spinning their space stations and sections of their spaceships.
In comparison, the Jellies—using tech scavenged from the Old Ones—were able to increase or decrease the inertial resistance of mass by increasing or decreasing the energy potential of the spacetime fluid around their spaceships and space stations. This not only allowed their ships to accelerate far faster than human ships for a given amount of thrust, it also allowed them to create true artificial gravity. This, plus the fact that Jellies were used to an aquatic environment, meant that the interior of their spaceships tended to be laid out completely differently than those of human spaceships.
A sphere of subluminal space permeated with a conditioned EM field that allows for tardyonic matter to transition through the membrane of fluidic spacetime into superluminal space. In other words, a protective bubble that allows a spaceship to travel faster than light.
Markov Drive (FTL)
Although long theorized, superluminal travel did not become a practical reality until Ilya Markov codified the unified field theory (UFT) in 2107. Empirical confirmation followed soon afterward, and the first working prototype of an FTL drive was constructed in 2114. This anti-matter machine made long jumps through space possible, allowing humans to venture into the stars. Markov became the most famous human scientist since Einstein.
Orb of Conversion
Jelly ships used this device to transition a ship from STL space to FTL space. The tech was scavenged by the Wranaui from Old Ones’ ruins and is essentially a more advanced version of the Markov Drive.