The phrase "to brush up" means to improve or refresh one's knowledge, skills, or appearance. It can be used in various contexts such as learning something new, practicing a skill, or preparing for an event. For example, someone might say they are going to brush up on their math skills before taking a test, indicating that they will study and practice math problems to improve their understanding of the subject. In another context, someone could be getting ready for a job interview by brushing up on their resume or practicing common interview questions. The phrase can also refer to improving one's physical appearance, such as brushing up one's hair or putting on some makeup before going out in public. Overall, the phrase "to brush up" means to take steps towards improvement and refinement.