y=mx+b M represents the slope, B represents the y-intercept, and x and y are points. Slope Intercept Form is just the equation of a line on a graph.
In the graph above, the slope of the line is 4/5 (rise over run for point m). To get the y-intercept (b), you take the x and y from one point on the line and plug them in to y=mx+b. Solve the equation to find b.