Instead of logging in to SSH using a traditional password, you can also authenticate yourself without a password using a technique called public key authentication. This has more advantages than you would expect: as well as being convenient and more secure, you could use it to allow applications on your computer to access a remote system without knowing anything about the authentication, and mount SSH filesystems without being prompted.
How does public key cryptography work? Two keys are generated on the client computer: the public key and the private key. What makes the two keys special is that a message encrypted with one can only be decrypted with the other.