The Animation Blueprint Editor is where you specify the logic the drives your character's animations and can be accessed by opening any Animation Blueprint from the Content Browser (or from the Editor Toolbar). This editor is very similar to working with the Blueprint Editor as it uses the same visual node based scripting system with an emphasis on animation features like managing animation State Machines, using Blend Nodes to control Blending Animations, or direct Skeletal Control through script.
For more information on working with Animation Blueprints and the Animation Blueprint Editor, refer to the Animation Blueprints documentation.