The Computer Officers provide a server to host cron jobs. These are tasks that are automatically run by the server at certain times or intervals. They are useful for doing things like regular maintenance and automatic build tests. While you can perfectly well set up cron jobs on a workstation, these get wiped out if the machine is reinstalled. It can also be tricky to remember exactly which workstation certain jobs are meant to be configured on.
cron.ch.private is not a powerful machine, so any cpu, memory, or disk intensive jobs should be set up to actually run elsewhere, for example by logging into the target machine using ssh public key authentication. This is still useful because if the target machine is down then cron will send you an email to tell you about it, whereas if the cron job was configured on the target machine it would fail silently.
For help setting up crontabs, have a look at the manpages on the cron server. 'man 1 crontab' and 'man 5 crontab' will tell you what you need to know. Watch out for the common gotcha that the PATH a cron job uses is not the same as your regular PATH.