Visa applicants need to show proof of financial means to finance their trip to and stay in Germany. In some cases, the applicant may be asked to present a commitment declaration(also called letter of financial support) signed by the sponsor who pays for the expenses of the visa applicant in Germany.
1. Students in Germany
In order to show that they have sufficient financial means for their stay in Germany, foreign students may be asked to present a formal letter of financial support (“Verpflichtungserklärung”) or to open a blocked bank account (Sperrkonto).
2. Visitors to Germany
For private visits, an invitation is usually required to confirm the purpose of the journey. In case the visitor is not able to fully cover the expenses associated with the trip a formal pledge to cover all costs (declaration of commitment) has to be presented with the visa application. In this case the host can sign the relevant form (“Verpflichtungserklärung”) at his/her residency's local authorities (usually Ordnungsamt or Ausländerbehörde).
Find more Information here (german)