About adaboost algorithm

I'm working on a traffic flow prediction where I can predict that a place has heavy or light traffic. I have classified each traffic as 1-5, 1 being the lightest traffic and 5 being the heaviest traffic.

I came across to this website about traffic flow prediction using adaboost algorithm, and I'm really having a difficulty learning this algorithm. Specially in the part where S is the set ((xi,yi),i=(1,2,…,m)). where Y={-1,+1}. what is x and y? and the constant L? what is the value of L?

Can someone explain me this algorithm?