The Services window lists all of the available services. You can add, change, remove, or pause services from this window.
Normally you probably won't need to modify the services. Simon comes with several pre-defined services, so you can just use them as-is. However, if you have a special type of service you want to monitor, the Services window allows you to add your own. Or if the default values for the services aren't to your liking, you can customize them to suit you.