SSH is your command line into the server. On macOS and Linux use the built-in terminal; on Windows use the built-in OpenSSH client in Terminal, or PuTTY.
The command is ssh root@your.server.ip. On the first connection you will be asked to accept the host key - that is normal, and it is your machine remembering the server so it can warn you if it ever changes unexpectedly.
Move to key authentication early. Generate a key pair with ssh-keygen -t ed25519, copy the public key to the server with ssh-copy-id, confirm that key login works, and only then disable password authentication. A server with password login enabled will see thousands of guessing attempts a day. With keys only, they all fail instantly.
Two more changes worth making in the same sitting: move SSH off port 22, and disable direct root login in favour of a normal user with sudo.
Do not lock yourself out. Keep your current session open while you change SSH settings, and test the new configuration in a second window before closing the first. If the new setup is broken, the still-open session is what saves you. If you do get locked out, our console access can rescue it - open a ticket.
