Answer:
There are two ways to think of a "career highlight". First, any person may have a "career highlight" in any job. (see below for explanation) Second, in highly professional careers, there may be specific milestones and honors.
Any person in any occupation can have a "career highlight". This includes persons working in any job from minimum wage or even volunteer work. For resumes and interviews, a "highlight" in this sense is the best work you ever received an acknowledgement for doing. Let's give some examples:
Patty worked her whole life in housekeeping at a large hotel chain in her town. She never missed a day of work and was honored for her loyalty and consistently excellent work. For Patti, this could be her "career highlight."
Samuel was a salesman. One year, he sold 40% more products than anyone else in the company. This may be his "career highlight."
But sometimes, we can choose which "highlight" we value more as an employee. These are often the highlights with the most emotional feedback. For example, Fireman Chuck rose to the rank of Captain. He supervised hundreds of men over the years and trained most of them. He received many commendations and honors. But, Chuck still considers one moment the highlight of his career, that being when he rescued a 2 week old baby from a burning apartment. The "baby" is now grown up, and now visits Chuck every year.
Dorothy went to her boring job every day just to feed her family. But she'll never forget the day she met a homeless lady who called Dorothy "an angel" because Dorothy didn't just ignore the woman's misery. Dorothy made sure to bring the woman cocoa and a sandwich every winter day, and cool drinks in the summer. Dorothy remembers that as her personal highlight because she could have easily just ignored the woman. But Dorothy was proud she, with little education or skills, could make a difference in someone else's life.
As hard as it was, Michael kept up his studies, determined to succeed. Even though he went on to earn honors as an Attorney, he still feels the highlight of his career was when he earned his Law Degree, upon which he built the rest of his life.
Professional work
Career Highlights for many workers means what honors they achieved-- usually the highest achieved. For one person, the highlight may be earning the title of Supreme Court Justice. For another person, it might be being named President of the Local Bar Association. A policeman becomes Police Chief.... A prior Senator becomes a US President. Etc.
Many people also name highlights along the way. These are often used as a resume of their career history. For example, a newly graduated teacher joins the staff at Some Elementary School; 5 yrs later, she's teaching at Other Place High School; 10 yrs after graduation, she's the President of the local Teacher's Union; she serves on the President's Task Force for Education; she was honored for Community Literacy Volunteerism; she became Vice-Principal at Other Place High School, but left to teach at A Nice University; Later she became Provost at A Nice University; she received dozens of community honors; she started her own Private College Prep School; etc.
Summary
So, "career highlights" may be one or a few significant moments that the person values the most.
Or, "career highlights" may be a series of accomplishments appropriate to list on a resume or Vita.