The phrase "to get active" means to become involved in an activity or to start taking action towards a particular goal. It can refer to physically engaging in some form of exercise, such as going for a run or joining a fitness class, but it also has broader applications that involve becoming more engaged mentally and emotionally. For example, someone might say "I need to get active" if they want to become more involved in their community or start working towards a personal goal like losing weight or improving their health.