Generation has finished and the program tells me it had to relax some constraints.

In case you have allowed constraints relaxation before the generation it is possible that you will see this dialog at the end of the generation:

What does it mean?
It simply means that computer was able to put all lessons into the timetable but it had to relax some of your constraints.

If you click the "Show me..." button the software will show you list of the constraints that it had to relax.

You can check if the constraints that were relaxed are realistic.

Also please check these articles: