A numerus clausus is a method in
Germany to restrict the access to a certain subject at a
university. The criterion is the grade of your
Abitur. This grade can range from 1.0 (best) to 4.0 (worst). If a subject has for example a numerus clausus of 2.2, then no one with a worse
Abitur gets in. They have to wait for next year (or even longer, if they are significantly worse). There is a waiting list for those who can not get in and every year there are positions hold up for those on the waiting list. Normally one year of waiting corresponds to 0.2 gradesteps. Though there are special cases when you can get in earlier.
If you are on the waiting list, you are not allowed to study another subject, because this does not count as waiting, but you can study on another university without nummerus clausus and change to this university later or you can study another subject which is closely related to the one you wanted to study and change to this subject later.
If you want to study in Germany you should check, if the subject of your interest has a numerus clausus (most of the interesting (ex. computer science, medicine, law or architectonics) have), because you have to sent in your application earlier. Most non-german school reports can be used, too.