What does the slope and y-intercept mean?

The y-intercept is the point on the line where x=0. In this case, it seems to be that when x=0, y=0, so the point is (0,0). The slope can be given by the difference in y values over difference in x values. Take two points that are on the line. In this case, it looks like there are points (2,6) and (10,30). 
(30-6)/(10-2) should give you the slope of that line, which appears to be approximately 24/5.

You need to show the box plots