Summary
apt-key is used to manage the list of keys(public) used by apt to authenticate packages. Packages which have been authenticated using these keys(public) will be considered trusted.
Note that if usage of apt-key is desired the additional installation of the GNU Privacy Guard suite (packaged in gnupg) is required. ref
Details
JAK
apt-key is the keyring. adding GnuPG key to the ring using command
add-apt-key - Command line tool to add GPG keys to the APT keyring
# or
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
# curl downloads the gpg key and apt-key add adds it to the keyring
apt-key list
to list of trusted public keys