Import GPG Key

I came across an issue today where I couldn't perform an

apt-get update

It threw an error saying something along the lines of "GPG Error. The following signatures couldn't be verified... NO_PUBKEY ..."

I googled and found this:

apt-key adv --keyserver keyserver.ubuntu.com --recv-keys <PUBLIC_KEY>

Where <PUBLIC_KEY> is the key in the error above.

Reference