That the warp drive functions through the expansion and contraction of space.
There were many possible fan explanations for how TOS' warp drive would work, with Miguel Alcubierre developing the Alcubierre drive as his own attempt at making warp drive real, but the in-show explanations never really delved beyond a "the warp engines make the ship go fast to a certain point, but too much, and they fly off the ship".
TNG's explanation only amounted to saying they worked by pushing on subspace, which pushed back and propelled the ship, like a detergent-powered paper boat being dragged along, and can be responsible for subspace funkiness depending on how you use it.
DS9 seemed to imply that they altered the mass of the ship in some way, in conjunction with the deflector systems. TNG only implied that the warp field could alter the laws of physics within its area of effect.
But at some point the "contraction and expansion of space" explanation for how a warp field works seems to have stuck, sometime after Enterprise(?). Ask any random Star Trek person about how warp drive works, and they'll give a similar explanation. The writers almost certainly operate under similar beliefs.