for (int x = 0; sets the starting point. Can be any value you want.
x<100; sets the ending condition. Can be any test condition you want.
x++) sets how x changes. Can be any change you want.
In this case, x is incremented by 1 each pass thru the for:next loop: x++ is same as x=x+1.
Say you wanted the loop to go from 102 to 112 by twos:
for (int x = 102; x<113; x=x+2)
Say you wanted to count down from 100 to 1:
for (int x 100; x >0; x=x-1)
So how you go thru the loop is up to what you want your code to do.
You can even change the value of 'x' within the loop to end it sooner.