A package is basically multiple objects grouped together, to form a single object. Packages can include anything from bricks to tools, and greatly enhance the ease of which you can manipulate a group of objects.
In addition, through creating a package, you are also given the option to upload it to Roblox. This feature allows any other player to access the package through using the All Models option within the Studio Toolbox or when browsing the Creator Marketplace.
For detailed information on how to create and manage packages, check the Roblox Packages article