What does the complexity of generation mean?

The complexity of the generation means how much time the computer shall spend checking your timetable.

Normal Complexity
This option is good for initial generations, where you want to check if your timetable is good inputted and generate-able.
For many schools this option will find a good timetable and they will have no need to use more complex and usually slower generations.

Large & Huge
This settings are more complex, the computer will spend more time checking your timetable. This can mean the generations will be slower. For some complex timetables it might be needed to use these complexities.

Notes:
- Please see this article: A good way to generate your timetable.
- Do not forget to test your timetable before generating. Why to test the timetable?
- It is not a good idea to use large and huge settings before you are quite convinced that there are no mistakes in the basic data and you have inputted realistic constraints. See also: Analyze the timetable by Extended tests

See also:
What is cloud generator?