When you click on the arrow, other options appear that you can try out. A screenshot of this feature is un-available at this time.

This is highlighting the Blend Mode of the selected layer. Altering the Blend Mode can make a huge difference in the way your layer may look.

This shows how opaque a layer is.

This icon allows you to lock the transparency of the layer.

This locks the layer of any brushing use.

This locks the layer of mobility.

This option combines the three previously mentioned locks to lock the selected layer.

This option is difficult to explain as it is similar to the opacity option. It will be explained more in-depth in a separate tutorial.

Clicking this icon will allow you to choose special effects to a layer and change the blending mode.

This adds a layer mask to the selected layer.

Selecting this icon is another method to add an adjustment layer.

This creates a group for you to place your layers in for a more organized feel. More details will be explained in a more detailed tutorial.

This creates a new layer above your selected layer.

This deletes your selected layer(s).

The blue highlights mean the layers are selected. You can select multiple layers by pressing 'Ctrl' and clicking on a layer or 'Shift' and clicking on different layers.

The green signals it’s still a text layer.

This layer is a text layer that has not been rasterized, which is basically converting vector to bitmap.

This layer is a regular layer with a layer mask attached.

This is a gradient map, one of the many Adjustment Layers that can be accessed through Layer >  New Adjustment Layer. Why does it have a white glow? That’s the layer mask. If an area is black, it will not show. If it’s grey, it will be semi-transparent. And if a part is white, it is opaque.

Layer 4 is the name of the layer. You can change it by double clicking on the name of the layer.

The eye shows the layer is visible.