the b is the y-intercept. (where the line meets on the y-axis) the m is the slope of the line. So now how you use it. Basically, if I ask you to graph the line y=2x+5 When you graph the line look at the b (y=2x+5) . Remember: The b is the y-intercept. Then you're going to make your first point (0,5) Then you have to do the slope of the line y=2x+5 Remember slope is Rise/Run. So the slope is 2/1. So make the slope 2. If you did this correctly your line should cross through the point (7,1)
Hope this helps! If you still don't understand then ask your teacher for more help!