The y-intercept is the value of y where an equation crosses the y-axis and x = 0. In the equation y = mx + b, if one substitutes 0 for x, the equation becomes y = b.
The form y = mx + b of a linear equation is called slope-intercept form because the coefficients m and b represent the slope and y intercept respectively. For any equation in terms of x and y, one can calculate the y-intercept by substituting zero for the value of x. In the case of a linear equation written in slope-intercept form, substituting zero for x results in y = m(0) + b, which simplifies to y = b. Therefore, b stands for the y-intercept.