During the relative refractory period of an action potential, a larger than normal stimulus is needed to cause another action potential. this is due to the fact that

Refractory period is the period after an initial stimulus when a neuron is not sensitive to another stimulus.
During the relative refractory period of an action potential, a larger than normal stimulus is needed to cause another action potential. This is due to the fact that the voltage gated potassium ion channels remain open long enough to hyperpolarize the axon membrane.