This is the equation of a straight line in the so-called "slope-intercept form". It is a useful form, although it cannot represent vertical lines (they have equations of the form `x=a`).
Here `b` is the y-intercept (a value of `y` for `x=0`) and `m` is the slope (the tangent of the angle of incline with respect to the x-axis).
The simplest way to draw a straight line is to find two distinct points on it and apply a ruler. One point is almost given, it has the coordinates `(0, b).` The second point may be obtained for, say, `x=1:` `(1, m+b).` The other way is to find the x-intercept, the point `x` where `y=mx+b=0.` For `m!=0` it is `-b/m,` so the second point is `(-b/m, 0).` For `m=0` the line is parallel to the x-axis.