go [ v] () layers block allows one to move a sprite between different layers, being in front or behind other sprites. Before Scratch 3.0, this block was in two parts: Go to Front and Go Back () Layers.
The dropdown in the block has two options: forward and backward. These allow you to either go forward or backward. Because of this, the block can be used in a wide variety of situations.
When a card is played:
when I receive [play card v] go to x: (0) y: (-120) go [forward v] (1) layers . . .
When setting the road up for a race:
when I receive [set up v] go to x: (0) y: (0) go [backward v] (2) layers
The second input changes how many layers the sprite moves forward or backward. Many scratchers need a sprite to go to the very front or back of the whole stage, so they use the
go to [ v] layer block instead.