The admission to a German university is valid for one semester only. For the following semester/s you have to actively "re-register". If you happen to miss the re-registration, you risk losing your university place.
To re-register you have to pay the student services fee (114,00 €). On the back of your student ID (which you receive at registration) you have the exact information about when, how much and to which bank account you will need to make the next bank transfer.
After you transfer the fee, the Registration Office will send you the student ID for the following semester, with which you will be re-registered.