A person who is friends with your friend and you don’t know them well enough to call them your own friend so they are your second-friend through association.

A second friend is a friend of your friend. They typically do not hang out with you unless you are hanging with your friend in common (known as a first friend). This is because hanging out with a second friend is usually awkward, as usually the only thing you have in common is that you both know your first friend. Sometimes if you spend enough time with a second friend, they can upgrade to a first friend. Then hanging out with that person is no longer awkward.