The Content tab (Hotkey Q) lets you quickly see the elements in your page or component. You can select and element by clicking it.
There are three types of elements you can add to your tree.
Standard HTML elements are shown with a blue icon. Text elements are shown in grey and components are purple.
You can add new elements by clicking the (+) button at the top (Hotkey E).
New elements will be added as children of the current selected element.
You can cut / copy and paste any element, using CMD + X, CMD + C, CMD + V. This works between components as well.
You can rearrange the elements in the tree by dragging them.
You can also copy elements by holding down alt when you drop them in the tree.