A program with preselection means that the process of sifting the applications is done by the admission committee of the respective degree prior to admission.
The selection process is based on among others academic prerequisites, professional and extricuricular experience as well as a selection interview.
Your application will be evaluated by the admission committee. Only after successful preselection your documents will be checked by the Master’s admission office with regard to the formal requirements. If your application meets the formal requirements you will officially be admitted into the program.