The equation y=mx+b is called the "slope-intercept" form of the equation of a line.
It is called so because if all the solutions (x, y) of this equation are plotted, they form a line with slope m and y-intercept b.
If m is positive, the line is increasing. So if m is positive, the greater the m, the more steeper the line, in increasing direction.
If m is negative, the line is decreasing. The smaller the m, the steeper the line, in decreasing direction.
b represents the y-intercept. If b is 0, the line passes through the origin. The larger the value of b, the higher the y-intercept. The smaller the value of b, the lower the y-intercept.