The Anim Preview Editor found inside of Animation Blueprints displays all the Variables and Variable categories associated with an Animation Blueprint. When you open any Animation Blueprint, you can find the Anim Preview Editor in the lower-right corner of the Animation Blueprint Editor. The Anim Preview Editor not only displays the variables associated with the Animation Blueprint, it allows you to adjust the values to see how it will affect any logic which may lead to changes in a character's animation state behavior.
Click image for full view.
Also included by default in this location on a separate docked tab is the Asset Browser window.
Variables created inside an Animation Blueprint can be assigned to categories for organizational purposes (a Default category is used if no category is selected).
You can type in a name for a category for the variable to be assigned to or select a previously created category from the drop-down menu.
Anim Preview Editor Modes
The Anim Preview Editor is split into two modes: Edit Preview and Edit Defaults
Edit Preview
The default option for the Anim Preview Editor is the Edit Preview mode which allows you alter variables that affect the preview Skeletal Mesh only.
Above, the option IsInAir? has been set to True and the preview mesh (based on the Animation Blueprint's logic) enters a falling state. Whenever you make changes inside the Anim Preview Editor and you are on the Edit Preview mode, those changes only affect the preview mesh and the changes you make will not be saved unless you click the Apply button which will take your values and assign them to the new default values.
You can also define the Root Motion Mode for the Animation Blueprint to use as well as define Optimization options.
Edit Defaults
The Edit Defaults option is very similar to the Edit Preview, however here you are editing the actual default values for your properties.
When you make changes to the values in this mode, you may notice the preview out of date warning in the viewport (which will go away once you click the Compile button).
The Public View option is a toggle that can be used to filter the variables to only show ones that have been marked as Public or all variables.
Above, the variables IsInAir? and Reloading have been defined as Public so only those are the only ones that appear in the Anim Preview Editor when Public View is enabled.