A blocked account (Sperrkonto) is a special account that is - in most cases - necessary for your entry visa application in Germany and is a precondition when applying for a residence permit.
If you are a foreign student, au-pair, language student or job seeker who is not from an EU Member State, you must prove that you have the financial resources to pay for your course of studies/stay in Germany as well as for daily life expenses during the time of your stay. It is usually required to provide this so-called 'Proof of financial resources' when you apply for an entry visa and is a prerequisite when you apply for a residence permit in Germany.
This proof of adequate financial means often requires a blocked account. Going more into detail, a blocked account is special account that is not freely accessible to the account holder upon opening of the account. Applicants must pay a minimum amount as required by law into the blocked account: currently, a minimum of 720.00 EUR has to be paid into the account for each month that the applicant plans to stay in Germany.
Note: embassies and consulates might, in some cases, request minimum blocked amounts that defer from the amounts required by law. However, usually the requirements for your blocked amount are as required by German law.
This 'blocked amount' that visa applicants need to deposit on the account will remain blocked until the account holder arrives in Germany. Another particularity of blocked accounts is that there are limitations on the amount that can be withdrawn or transferred by the account holders within a defined period of time. At the moment, account holders only may withdraw a maximum of 720.00 EUR per month from their account, unless they have paid in more than the demanded minimum blocked amount.