Ur career highlights refers to the peaks and/or the pivotal points of ur career. It is a modified CV that lists ur accomplishments in particular jobs. It is usually used if u have had extensive work experience so u can can just pick on what achievements u were able to make in the particular jobs u have had.
If your experience is limited, list all the achievements u have made in any job u've held. Make it sound good. That's u are being asked for a career highlight is a positive step.
Wishing u the best.