From encyclopedia Britannica:
"After repolarization there is a period during which a second action
potential cannot be initiated, no matter how large a stimulus current
is applied to the neuron. This is called the absolute refractory
period, and it is followed by a relative refractory period, during
which another action potential can be generated, but only by a greater
stimulus current than that originally needed. This period is followed
by the return of the neuronal properties to the threshold levels
originally required for the initiation of action potentials."
Absolute refractory period = initiation impossible absolutely
Relative refractory period = initiation possible but in greater energy relative to the original required.