The Rober problem consists of a stiff system of 3 nonlinear ordinary differential equations describing the kinetics of an autocatalytic reaction given by Robertson (1966).
The large difference among the reaction rate constants is the reason for stiffness.
As is typical for problems arising in chemical kinetics this special system has a small very quick initial transient. This phase is followed by a very smooth variation of the components where a large stepsize would be appropriate for a numerical method.
A complete description of the problem could be found in
rober.pdf