is the intercept (where the line crosses the y-axis)
Recall that the slope (m) is the "steepness" of the line and b is the intercept - the point where the line crosses the y-axis.
In the figure above, adjust both m and b with the sliders to see the effect of these variables.
Equations of this type that have no exponents in them (such as x2) are called 'linear equations' because they always graph as straight lines. The word "linear" is derived from "line".