Every Debian package has a priority associated with it, indicating how important the package is to the system. The priorities are:
The package is essential to the proper functioning of the system.
The package provides important functionality that enables the system to run well.
The package is included in a standard system installation.
The package is one that you might want to install, but you can omit it if you are short on disk space, for example.
The package either conflicts with other packages that have a higher priority, has specialized requirements, or is one that you would want to install only if you need it.
The control file for dpkg, for example, shows that dpkg itself has a priority of required; dpkg-dev (which provides tools for building Debian packages) has a priority of standard; and dpkg-doc is optional.