SSH-Keys

https://www.atlassian.com/git/tutorials/git-ssh

SSH keys are generated through a public key cryptographic algorithm, the most common being RSA or DSA. 
At a very high level SSH keys are generated through a mathematical formula that takes 2 prime numbers and a random seed variable to output the public and private key. 
This is a one-way formula that ensures the public key can be derived from the private key but the private key cannot be derived from the public key.

You can consider the public key as the lock and private key as the "key" to unlock the lock.

Generate keypair

Example

ssh-keygen -t rsa -b 4096 -C "[e-mail]"

ssh-keys1.png

This will create two files, the id_rsa which is the private key and id_rsa.pub which is the public key. This is the key you want to share with other hosts.

Add the public key to .ssh/authorized_keys

Ubuntu

The public key would go into /home/USERNAME/.ssh/authorized_keys