All Macs include something called launchd in the system which can schedule things. But it can be quite complicated to configure it as it requires complicated text files formatted exactly to some rules. Lingon takes care of all this for you and presents a simple interface where you can fill in what you want to do and then it is taken care of in the background.
In Lingon you can see all configuration files that exists in the system. And if you need to make a configuration that is not included in the interface you can switch to a text mode and with some help include all configuration possibilities.