The apartment deposit (Kaution, Mietkaution or Mietsicherheit) is an amount you give to your landlord when you sign an apartment contract. When you leave the apartment, this amount will be returned to you, with interests. If you broke something in the apartment, the landlord can use the money from your Kaution to fix it
In Germany, the Kaution is usually 3 times your monthly rent (Kaltmiete). This is the legal maximum.