They had spaceships that could travel sublight using normal acceleration, but could “jump” between stars, but only at the “Alderson point” - apparently a “wormhole” that existed between stars that their “Alderson drive” could make use of.
“Throughout the past thousand years of history it has been traditional to regard the Alderson Drive as an unmixed blessing. Without the faster than light travel Alderson’s discoveries made possible, humanity would have been trapped in the tiny prison of the Solar System when the Great Patriotic Wars destroyed the CoDominium on Earth. Instead, we had already settled more than two hundred worlds. “A blessing, yes. We might now be extinct were it not for the Alderson Drive. But unmixed? Consider. The same tramline effect that colonized the stars, the same interstellar contacts that allowed the formation of the First Empire, allow interstellar war.
The worlds wrecked in two hundred years of Secession Wars were both settled and destroyed by ships using the Alderson Drive. “Because of the Alderson Drive we need never consider the space between the stars. Because we can shunt between stellar systems in zero time, our ships and ships’ drives need cover only interplanetary distances. We say that the Second Empire of Man rules two hundred worlds and all the space between, over fifteen million cubic parsecs.
“Consider the true picture. Think of myriads of tiny bubbles, very sparsely scattered, rising through a vast black sea. We rule some of the bubbles. Of the waters we know nothing. .
Great book I'm re-reading now.