The element tree lets you view all the elements in the page or component you are editing. Just like with the canvas you can select elements by clicking on them.
Right clicking an element in the tree will open a context menu that lets you perform several different actions such as:
You can add elements to your page or component by selecting "Add element" from the context menu or by clicking the button in the bottom of the panel.
This opens the element catalog where you can browse different types of elements.