CSS translateZ(n) moves an element from its current position towards z-axis but x-axis and y-axis remains same from its current postion. You can compare image 1 and image 2 to see the differences.
Here Image 2 is moved towards 50px z-axix but x-axis and y-axis are remained same.
Image 2:
Here, in Image 2, Image moves 50 pixels towards z-axis from its current position.