Design System is a set of detaily created UI elements sets and patterns which are used in interfaces. It’s an important part of every UI as it drives the brand's image and the attention of a user on common elements. So it gets easier for every user to adjust to the platform or service faster and intuitively get to the desired point of the interface without huge efforts. Basically the design system is a set of buttons, navigation bars, colors, fonts etc, which are repeatedly used in the design.
Hello! If you are reading this, be aware, I am super happy to see you here and hope that you will become an incredible designer! Once you are planning to start your learning path in the UX/UI field, the first thing to do is to get more knowledge about what exactly UX, UI and the differences between these two branches of design. I would strongly recommend looking into this article first and then get back here, so I can guide you through your UX/UI learning journey!