What is a cron job?
A detailed explanation about crons is given in the wikipedia at http://en.wikipedia.org/wiki/Cron
Long story short, a cron is a task that runs at regular intervals on your linux server.
Being a task, it means that at the time it runs, it will do an action and in mailwizz case, this means it will send campaigns, process bounces, do cleanup and so on.
Why do i need to add them?
The reason why Mailwizz uses cron jobs is very simple, the tasks that it runs, to send campaigns for example, are tasks that can take a long time to execute and if you run these tasks as a regular web page, then not only we would need to interact with these pages in order to trigger the actions, but also it would mean we need to keep those pages open as long as they execute and that the execution might fail for a dozen of reasons (say script timeout, or memory limit, etc). Using cron jobs, all these actions are handled without our interaction and can run for unlimited time without the need of interaction.
How do i add them?
The bright side is that, control panels like CPanel or Plesk will give you a web interface to add your cron jobs easier.
Youtube has a bunch of videos for CPanel, Plesk , DirectAdmin and others, so there shouldn’t be a problem adding yours no matter what control panel you are using.