SSH means "secure shell".
a shell is the external part that the user knows of a system.
with secure shell its users can operate a system securely even when there is an untrusted
information network between the user's local system and the remotely accessed system (e.g. accessed through internet)
ssh forms a layer of protection with encryption over the network use of a shell such as bash shell.
ssh has many other uses such as tunneling and x11 forwarding.
see also: bash: a popular free textual shell for unix-likes, also scriptable