This step by step example should explain it best:
- You create a project with 3 layers.
- You select the top layer.
- You hold SHIFT and press arrow DOWN.
- Now you've got 2 layers selected, the 1st and 2nd layer.
- You keep holding SHIFT and press DOWN again.
- Now you've got 2 layers selected, the 2nd and 3rd layer selected.
- You create a project with 3 layers.
- You select the top layer.
- You hold SHIFT and press arrow DOWN.
- Now you've got 2 layers selected, the 1st and 2nd layer.
- You keep holding SHIFT and press DOWN again.
- Now you've got 3 layers selected, the 1st, 2nd and 3rd layer selected.