Let's go back to that pizza that's cut into 100 pieces...
With fractions, the whole pizza would be all 100 pieces:
So, the whole thing would be 100%!
What if you had 

pizzas? 
What percent would that be?
Cut each pizza into 100 pieces...
That's 350 pieces... So, that's 350%!
Sometimes, you'll hear someone, like a coach, say, "I want you to give 110% out there!"
If you tried your hardest, that would be your whole effort, right? So, that would be 100% of your effort. Doing more than that would be impossible. But, that's just their way of saying that they want you to try harder that you ever have before.
In the next lessons, I'll show you how to bounce back and forth between percents, fractions and decimals. It's pretty simple stuff and you'll soon be a pro.