update is used to resynchronize the package index files from their sources. The indexes of available
packages are fetched from the location(s) specified in /etc/apt/sources.list. For example, when using
a Debian archive, this command retrieves and scans the Packages.gz files, so that information about
new and updated packages is available. An update should always be performed before an upgrade or
dist-upgrade. Please be aware that the overall progress meter will be incorrect as the size of the
package files cannot be known in advance.