What is the best boy do?

I was reading an online article about "gaffer' and I stumbled upon the term "best boy". I am a bit confused because I did not find any further details, this term was just mentioned in the article. 
So what exactly does the best boy do on the movie set? What is their purpose?

You know that on the set, you need to set up the lights. The best boy makes sure that everything goes according to plan and is the first assistant of the grip crew