The define layers dialog lets you modify any layers you've created. You'll see a list of all the layers currently in your document, their visible and clickable settings, and the number of objects they contain.
To select a layer, click on its name. This will highlight the layer's properties and the number of objects it contains.
You can now rename the layer by clicking on "Edit Layer."
You can change its clickable and visible properties either under "Edit Layer" or by simply checking or unchecking the relevant boxes.
Selecting Visible allows your layer to be seen even when viewing a different layer in your visual.
Selecting Clickable allows your layer to be edited even when the layer is not selected and you're on a different layer of your visual. For example, you can select objects in that layer and move them, and so on.
You can also move the selected layer up and down in the order.
Clicking on "Remove Layer" will delete the selected layer.