Glossary
SSH key
A key pair that replaces passwords for remote login.
An SSH key pair is a private key you keep and a public key you install on servers. Logging in, your client proves possession of the private key by signing a challenge — the key itself never leaves your machine, which makes it both safer and more convenient than a password.
Generate with ssh-keygen -t ed25519, install with ssh-copy-id user@host. Permissions matter: SSH refuses private keys that others can read, hence chmod 600 on the key and 700 on ~/.ssh.
See it in practice
$ ssh-keygen -t ed25519 && ssh-copy-id user@serverCreate the pair once, install the public half on each server. Passwords stop being part of your workflow.
Where you'll meet it
Log into remote machines, run remote commands, use keys instead of passwords, and tunnel ports.
chmodChange file permissions with numeric (755, 644) and symbolic (u+x) modes — clearly explained.
File permissionsWho may read, write and execute — nine bits per file.