When referring to a two and three-dimensional plane, a y-axis refers to the vertical height of a two or three-dimensional object. In the picture to the right you can see the y-axis plane going up and down and intersecting with the y-axis and z-axis.
A good example of where the y-axis is used on a computer is on the mouse. By assigning a value to the y-axis, when the mouse is moved up or down increasing or decreasing the y-axis value will allow the computer to know where the mouse cursor is on the screen.
When referring to a two and three-dimensional plane, a y-axis refers to the