Asset packages are collections of files and data from Unity projects, or elements of projects, which Unity compresses and stores in one file with the .unitypackage extension. Like a zip file, an asset package maintains its original directory structure when it is unpacked, as well as metadata about assets (such as import settings and links to other assets).
To create your own (custom) asset package, use the AssetsAny media or data that can be used in your game or project. An asset may come from a file created outside of Unity, such as a 3D Model, an audio file or an image. You can also create some asset types in Unity, such as an Animator Controller, an Audio Mixer or a Render Texture. More info
See in Glossary > Export Package menu option to compress a collection of assets in your current Unity project. For more information, see Creating your own Asset packages.
To import a local custom asset package into your project, use the Assets > Import Package menu option to unpack the collection into your currently open Unity project. For more information, see Importing Asset packages.
Re-install Standard Assets, Upgrade Standard Assets, Upgrading Standard Assets, Install Standard Assets, Installing Standard Assets, Import Standard Assets, Importing Standard Assets