As an SSH user you need a key pair consisting of your public and private keys. As the names indicate the public key is not secret, and can be shared freely. The private key is secret and usually personal, and should be stored and transferred so that it’s only ever accessible by you. If you already have a key pair it will probably be stored as ~/.ssh/id_rsa (private key) and ~/.ssh/id_rsa.pub (public key).
The “rsa” part of the name is the name of the main algorithm used to create the key. Several other algorithms are available, but the filename should start with “id_”.
If you do not have a key, creating one with default settings is as simple as running
ssh-keygen and following the instructions. An example session: