There are multiple definitions - click on the translation that matches your context.
Auftrag means:
Substantiv
partial
noun
In the United States, the term "retainer" is often used interchangeably with "retainer agreement", which the formal document under which client engage his professional service provider (in most cases, a lawyer) and the terms and conditions of the engagement are stipulated.
an advance fee paid to a lawyer or other professional by a client. As the lawyer works on a matter, he or she will bill time against this fee.
Due to the expense of litigating this case, our attorney has requested a large retainer up front.