Ok y is your y value from any point on the graph m is your slope x is the x value of any point from the graph and b is your y intercept that is where the line cross in the y line that is the vertical line in the graph
As simple as possible, it's a form of linear equations.
The formula, y = mx + b, can be dissected to get the slope and y-intercept of a linear equation where:
m = slope
b = y-intercept
(x and y can be substituted for any point)
If you have any more questions, feel free to ask. Hope this helps!