Many software packages for parallel processing on a network of workstations, for example MPI, make use of Unix's rsh command. This Web page will tell you what you need to know about this.
The name "rsh" stands for "remote shell," meaning that it will execute a shell command at a remote machine. For example, if you are on the machine hp8.cs.ucdavis.edu and type
rsh hp10 ls /tmp
this will result in hp8 sending a message to hp10 to run the Unix ls (on the directory /tmp) command there are hp10 (and send the output back here, at hp8).
In order to do this, you need to tell hp10 that it is OK to accept commands from you at hp8 without asking for a password. You can do this by including a line
in a file ~/.rhosts in your home directory at hp10.
(Actually, in this particular case, the machines hp8 and hp10 happen to share a common file system, so that .rhosts file will be common to both hp8 and hp10. By the way, they will have separate /tmp directories.)
In your ~/.rhosts file, make sure to put each machine name on a separate line, and do use fully-qualified addresses, e.g. hp8.cs.ucdavis.edu instead of just hp8. In some cases, you may need to include both hp8 and hp8.cs.ucdavis.edu.
MPI etc. call rsh in order to initiate the running of the given application program at several different machines, instead of the user having to launch them "by hand" at each machine.