There are lots of different things you can reflect on:

Reflect on any feedback you receive - why have you received it? How can you address it? What changes do you need to make?
Reflect on the way you study and whether it's working for you - have you found your best time of day, most efficient methods, preferred learning style?
Reflect on any practical exercises you carry out - could you have approached them in a different way?
Reflect on the way you deal with things, including your feelings, reactions, and your own strengths and weaknesses.
Reflect on how the skills you're learning at unviersity (and through extracurricular activities) are transferrable to the work place.

Hint
Don't forget to focus on what you're doing right, as well as what you can improve. Reflection should be positive as well as constructive.

Hint
Reflection should be all about WHY and HOW, not just WHAT. Why was something successful/ not successful? How can something be improved or developed?