Explain Basic Supply Demand Graph
Assuming that $W$ is wage and $L$ is labor, I would say this is a labor supply-demand graph. TD stand for demand and S for supply. 

The graph shows a positive shift in labor supply, i.e. labor offered by workers, leading to an increase in $L$ and a decrease of $W$ due to the excess of supply. 

The shift in labor supply can be caused by a variety of shocks as a preference shock which values more labor, a change in non-labor income, etc (look here for explanations)