When you apply for an apartment in Germany, you must show a proof of income.
If you don't have a steady income, you can ask a parent or a friend to be your guarantor. If you don't pay the rent on time, your guarantor will have to pay it. This is a good option for students, freelancers and people with a low income. This is called a Bürgschaft or Mietbürgschaft.
The Bürgschaft letter has no specific format. If your mother or father is your guarantor, use this Mietbürgschaft template.