Content hot reloading allows you to reload your already existing Unreal assets that may have changed on disk while the editor is open. With this feature you do not need to restart the editor to reload your content from disk. The most common use case for this is syncing new content from source control. Previously you had to restart the editor for your content to update.
Auto-Reimport detects changes to import format files such as FBX for meshes, or png for textures, and update the asset from source.