The phrase "to loosen up" means to become more relaxed or less tense. It can refer to a physical state, such as when someone is told to loosen their grip on something, or it can refer to a mental state, such as when someone needs to loosen up and not take things so seriously. The phrase can also be used figuratively to describe the act of becoming more relaxed in other areas of life, such as by taking time off work or letting go of stressful situations.