SSH uses public/private key pairs.
id_rsa is your RSA private key (do not share this!).
id_rsa.pub is your RSA public key; this you give out to the administrator of the target system so that they can add it to verify that the signature came from your private key.
On your source system, these 2 files are located under under the
.ssh folder. For example:
To connect via SSH:
ssh -i /home/oracle/.ssh/id_rsa firstname.lastname@example.org
To connect via SFTP:
sftp -oIdentityFile=/home/oracle/.ssh/id_rsa email@example.com