The Attribute Editor is one of the central windows enabling you to load the UIs for each Element in your project in order to manipulate attributes, set keyframes and make connections.
Multiple UIs can be loaded at the same time giving you a global view and providing access to multi actions. This is an ever changing window depending on your preferred workflow.
An Element's UI will appear in the Attribute Editor on creation. There are several ways to load Element UIs into the Attribute Editor.
Drag an Element into the Attribute Editor from the Project Window.
Double click on an Element in the Project Window (except for Comps - double click will take you into the Comp).
Turn on Live Mode in the Attribute Editor. With Live Mode on, selecting an Element in the Project Window will clear the Attribute Editor and only load the selected Element's UI.
To remove an Element's UI from the Attribute Editor click the 'x' icon in the top left of the title bar.
If you have a particular Element you're working with a lot you can make it persistent by pinning it to the Attribute Editor. Set the pin icon in the title bar to its green state to prevent the UI from being removed (e.g. in Live Mode or when alt > double clicking an Element in the Project Window).
To learn about how to interact with the attributes see Control Rows.