How powerful would they be ?
The Transcension hypothesis attempts to answer the Fermi paradox by saying that sufficiently advanced civilizations nearly invariably leave their original universe for one of their own making. By definition, a transcended civilization would have the power to create or manipulate new universes or self-enclosed pockets; this would likely require a very advanced understanding of physics. This understanding would probably be matched in other sciences.
This is my impression from a few minutes of searching. I do not know why you asked the question of “what it is” when a simple search would have been faster. I do not expect that many people here are very knowledgeable about this particular hypothesis, and this is a basic question anyway.
The hypothesis does not seem very likely to me. It claims that transcendence is the inevitable evolutionary result of civilizations, but in nature we observe many niches. Civilizations are less like individuals in a species, and more like species themselves. And since a single civilization can colonize a galaxy, it would only take one civilization to produce a world unlike the one we see today - there would have to be not only no other niches, but no mutants either.