SSH is an acronym for a Secure Shell. Bassically this is an Internet protocol for establishing remote access with a secure type of connection. Via the SSH connection you can easily edit the content of your files, move them, create folders, copy files and many more usefull features.
SSH key is a key pair which is usualy required for the key based authentication method over SSH connection. This type of authentication provides you with even more secured connection that the usual used password type of authentication.
The key pair can be generate using the PuTTy Key generator software. This type of connection noramlly requires a passphrase which is a password on the key. The key pair consist of two keys. The first one is a public key and the second one is a private key. The public key is usually kept on the server to which you are connecting and the private key is kept on your local computer.
If some of these keys is missing the connection will be aborted and you will be unable to connect to the server.