The y- axis (the vertical axis) acts as a mirror point. When reflecting over the y axis, the y value will stay the same. Think about looking into a mirror on your wall, your height is the same on both sides of the vertical reflecting object
The x-value will change and be the other side of the y-axis. It will be the same distance away from the y-axis that is was, but will have an opposite sign.
Here is was 6 units to the left of the y-axis (where x values are negative), now it is 6 units to the right of the y-axis (where x values are positive)
