Many universities or universities of applied sciences in Germany offer so-called preparatory courses or preliminary courses. The goal of the preliminary courses is to help you make the transition from school or language school to university, to close any existing gaps in your knowledge - and to do so before the start of your studies. Participants have the opportunity to become acquainted with their new field of study. You will also meet your future fellow students. Preliminary courses usually take place shortly before the beginning of the semester. And best of all: the preliminary courses for first-year students are usually free of charge.
For one thing it is interesting to gather first impressions at the university. In the preparation courses, school material is refreshed in compact form through lectures or tutorials. What one should know in principle is supplemented. The entire basic material is trained so that you are fit for the start of your studies. The respective student councils also often offer an orientation to make it easier for students to get started. So you can personally get to know your future fellow students as well as your new hometown.
Knowledge of math is especially essential for first-year students in the natural sciences. The math one learned in school is often not sufficient. Depending on the subject of study, courses are offered at varying levels with different content.
If you want to study computer science, you either already have some programming knowledge or not. In any case it is a good idea to expand your knowledge in a preliminary course. There are universities that offer preliminary courses in computer science, often in cooperation with their respective student councils. Roughly speaking, these courses involve programming principles that apply to all programming languages: variables, branches, arrays, loops, methods, object orientation. These are practiced using a programming language, e.g. Java.
Preliminary courses are generally a voluntary offer of the university/university of applied sciences to make the transition and start of your studies easier for you. You can take advantage of this offer if you choose, but you don't have to. Maybe you attended a preparatory college (Studienkolleg) before you started your studies and, thus, obtained the academic level of a German high school graduate.
As a foreign student, you will usually be prepared for your studies in Germany in a preparatory college (Studienkolleg), both linguistically and subject-wise. Some universities also offer special language courses. If you haven't had to attend a preparatory college or don't feel 100% fit yet, a language course and a preliminary course that focuses on a particular subject are certainly not a lost investment.
Remember: If German is a foreign language for you, you must have a certain language level at German universities in order to be able to study. For this you need the German Language Diploma of the Conference of Ministers of Culture (DSD) Level 2. More about language tests