We often have little control over what happens to us. What we do have control over though is how we respond. Do we respond with resilience (bounceback-ability) to setbacks, do we respond with anger or coolness to provocation...
Through our habits we often have set natural responses to how we react. With setbacks we often naturally sulk, with provocation we naturally react angrily. Proactivity is how we disrupt our natural or habitual response and act in a way that aligns with our values.
Proactive people take matters into their own hands and take the initiative in their response.
Examples of where you can be proactive:
If you do not understand something in a class a proactive person would ask the teacher or look it up either during or after the class.
If you are looking to get into a particular university course consider how you could make your application stand out - Potentially through volunteering or completing certain extra-curricular awards/activities
Starting revision earlier than normal so that you can do it in short small chunks.
This is not an exhaustive list, proactivity is a habit that can run throughout every aspect of your life.