ssh or Secure SHell is used to facilitate communication between processes. In order to pass messages and to spawn processes, ssh is used non-interactively to pass on UNIX commands to each node. ssh addresses different security-related issues as it encrypts all communication between nodes. This form is used simply because the code is run on a public network (as opposed to private ones which is not reachable by computers connected to the internet) to prevent any system breaches for other melicious purposes than just running and testing parallel codes.
For Linux users, OpenSSH implementation is commonly used, and this is what we will use for this setup.