y = mx + b is the slope intercept form of a line. m is the slope, b is the y intercept.
y = 3x is a line with a positive slope. It angles up away from the x axis.
If the slope is 0, the equation is y=0x which is y=0.
That graph is the x-axis.
Below, y=3x is the red line. y=0 is the green line.