In Unreal Engine, animation Pose assets are stored skeletal mesh poses that you can use in your project, as an animation target or reference point.

Pose assets save the position of the mesh geometry as well as the skeleton data. Pose assets can contain a single static pose, or many poses, saved as animation curves within the asset.
Creating Pose Asset
There are many ways to create an animation Pose asset.
While working in the Skeleton Editor, skeletal mesh editor, or the animation sequence editor, you can create a pose asset, using the create asset button in the animation editor Toolbar to save the current skeletal mesh position as an animation Pose asset.

You can also create an animation Pose asset from an entire animation sequence by right-clicking the asset in the Content Browser and selecting Create > Create PoseAsset from the context menu.

After selecting Create PoseAsset, the Create Pose Asset window will open, where you can select which animation sequence you wish to create an animation pose asset from.

When generating a Pose asset from an entire animation sequence, Unreal Engine will create a pose for each frame of animation. The generated poses will then be accessible using animation curves present in the Pose Asset.
You can sequentially input any desired names for the generated pose curves by typing a name on a new line for each curve or frame of animation, in the [OPTIONAL] Pose Names field.

Additive Pose Assets
After creating and opening a Pose Asset, you can modify a pose asset, to behave as additive by toggling the Additive property and selecting the Convert to Additive Pose button in the pose asset's Asset Details Panel.

Additive pose assets are able to play in an additive capacity, in conjunction with other animation data, such as animation sequences, without overriding the entire pose.
Using Pose Assets
Pose assets can be used in many ways to animate characters.
Pose Blender
You can use the Pose Blender and Pose Driver animation blueprint nodes, to control the playback and blending of animation Pose assets to animate characters at runtime.