mPezcount=newAmount why we use this ?

Originally in the beginning of the video the load() method would just add a pez to the total amount each time you called load.

public void load(int pezAmount){
     mPezcount += pezAmount;
}

+= indicates is the same as saying : mPezCount = mPezCount + pezAmount;

This was incorrect to do since the pez dispenser can only hold so many pez. What the instructor does is add a if statement which throws and exception ( something that lets you know something is wrong) to let you know if the pez dispenser is full. In the new code that you see mPezCount isn't being set. Instead newAmount is being set to mPezCount + pezAmount;

Next the method checks to see if newAmount is more than the maximum allowed by the dispenser.

If the code doesn't throw an exception then we get to the bottom line of code which you asked about which just does what we were doing originally

mPezcount = newAmount 

which is equivalent to

mPezcount = mPezCount + pezAmount;

which is equivalent to the original code

mPezcount += pezAmount;

To sum it all up, the new code that is written in the load method just checks to see if the total amount of pez will be too high before assigning it to mPezcount. Hope this helps.