Refractory period

I don’t understand the last few lines on the left side of the book. “After this there is a period...”


On the depolarization repolarization graph , the threshold doesn’t look like it raises

There are two parts to the refractory period. You have the absolute refractory period during which an action potential cannot be fired under any circumstance, and the relative refractory period during which an action potential can be fired but it is more difficult.
The absolute refractory period occurs because when sodium channels close (at the end of depolarisation) they also become inactivated and therefore cannot open to fire an action potential. The only way for the sodium channels to reactivate is by repolarising the cell. Once the cell has repolarised a bit, more and more sodium channels become reactivated as the membrane potential becomes more negative.
Once the sodium channels start to reactivate, it is possible to fire an action potential, but it would require a larger stimulus (as not all of the sodium channels have reactivated yet). Also, when the cell becomes hyperpolarised (due to delayed closure of K+ channels), a larger stimulus is required to depolarise the membrane sufficiently to reach the threshold potential - therefore making it more difficult to fire an action potential.

Ohh thanks a lot .
just to make sure I understand this, I don't know if I understand what "action potential" really is. I mean I understand that when the neuron is depolarised it fires an action potential and what happens with the Na and K ions and their channel proteins but I'm still not sure about what is meant by action potential.
Is it the potential difference when the neuron is depolarised ? If so, then what do we mean by an action potential is fired ?

An action potential is just another term for an 'electrical impulse.' Its the wave of depolarisation that moves from one side of the axon to the other. Remember that the membrane must be depolarised to the threshold potential (around -55 mV) in order for an electrical impulse/action potential to be fired - at this point, voltage-gated sodium channels open and the impulse is conducted along the axon.