Have you ever heard of React (JavaScript Library)
i had not heard about it but learned it is a javascript library for building user interfaces
What does it mean to build a user interface?
its like a way the human and computer interacts, like how your screen looks and where you click on pages 
So is JavaScript a computer language for that?
yes its one of the three core technologies along with html and css on the internet 
Which one is the easiest to learn?  I would love to learn one.
i do not know actually, but i do know html is the standard for creating web pages
So if I want to build websites html is the best?
yes i believe so but if you want animations or applications you will need adobe flash