Under the Link tab of the layer, you can specify a link which will be appear for the current layer. So if you click on any point of the layer, then the link you set will be loaded. You can also specify the target of that link with the selectbox, right under the link url. You can also set some additional parameters for the current layer. It can be useful for example when you would like to add the alt tab for the image, and also for SEO purposes. You will see after each line that which parameter refers for the current element or for its parent one.
By adding title to a layer then it will display as a tooltip if someone holds his mouse cursor over the layer.
You can add an Alt (alternative text) to your layer which is indexed by search engine robots and it helps people with certain disabilities.
Some plugin may use the Rel attribute of a linked content, here you can specify it to make interaction with these plugins.