Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

As we learnt from page 119, $m$, the coefficient of x, is the slope or gradient of the linear graph. It is the average rate of change of y with respect to x. $b$ is the y-intercept of the graph. It is the value of the graph when $x=0$, or the point where the graph intersects with the y-axis.