An asset is a set of nodes that are organized inside a special node called a container and fulfills a set of requirements outlined by a template. Some of the things you can do with assets are:
Nodes contained in an asset are called internal nodes while the container itself encapsulates them.
With containers you can create templates to plan and organize a scene to define the capabilities and attributes of various parts of the scene. Conversely, you can set up containers and publish attributes as you create your geometry and then save a template based on that. Either of these methods is key to setting expectations for future artists working on the asset.
Assets are particularly useful for streamlining and maintaining workflows and allows the scene author to control what aspects of the scene particular artists are allowed to modify. For example, a rigger working on a character can create a container with an interface of attributes needed for animation. When the model is sent down the pipeline, the animator only sees the attributes that are keyable and exposed for the animation department. This simplified subset of attributes prevents the animator from having to search through long lists of non-animation attributes.
Containers are also useful when parts of the model need to be swapped in and out. For example, multiple versions of arms and legs for a robot can be stored in their own files and referenced into a scene using containers without breaking existing hierarchies and animation (For more information see Reference an asset with file referencing).
