SSH stands for Secure Shell. The term "SSH" can refer to both the SSH protocol and the software tools that enable system administrators and users to establish secure connections to remote computers using this protocol.
The SSH protocol is an encrypted protocol designed to provide a secure connection over an insecure network, such as the internet. In Linux, SSH is based on a portable version of the OpenSSH project. It follows a classic client-server model, with an SSH server accepting connections from SSH clients. The client connects to the server and displays the session to the remote user, while the server accepts the connection and executes the session.
By default, an SSH server listens for incoming connections on Transmission Control Protocol (TCP) port 22. Since this is a well-known standardized port, it is often targeted by threat actors and malicious bots.