The Limit of Questions is when there are too manny intelligent lifeforms (or too many observers) on a single planet, which could lead to a singularity that would destroy the Universe.
(From episode 46)
Scub Corals are making information copies of everything that exists on the planet, so when the Limit of Questions is reached, they will go to another universe on the other side of the singularity when the material plane collapses.
(From episode 40)