Both equations are in the form $y = ax + b$. Since both have the same value for $b$ (which is $1$), they share the same y-intercept. This means both lines cut the y-axis at the same point, $(0, 1)$.

However, they differ in their slope because the value of $a$ is different. The equation $y = 3x + 1$ has a positive slope ($a = 3$), causing the line to rise from left to right. In contrast, $y = -3x + 1$ has a negative slope ($a = -3$), causing the line to fall from left to right. Changing the value of $a$ while keeping $b$ fixed changes the slope of the line.