To visit Germany, one often has to prove that they can pay for their trip and stay in Germany. When someone cannot do so, a third person can make a so-called "declaration of commitment" for them. With such a declaration, the third person undertakes to bear the costs for the visitor(s) in an emergency, including costs of medical treatment or possible deportation.
If, for instance, friends or family members of yours want to apply for a visa for Germany but cannot prove that they can pay the expenses for the trip and their stay, you can submit a declaration of commitment for them.