CSS translateY(n) moves an element from its current position towards downward (i.e, towards y-axis) but x-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 downward but x-axis is remained same.
Image 2:
Here, in Image 2, Image moves 50 pixels to the downward from its current position and x-axis is as it is.