Shows a person's strong determination or resolve towards achieving something. He's set on becoming a doctor, no matter how hard the studies are.
Pro Tip 2/3
No Alternatives
Indicates that someone is fixed on a single course of action without considering other options. She's set on starting her own business, even though her parents advised against it.
Pro Tip 3/3
Potential Opposition
Can imply a person's decision may face opposition or is contrary to advice given. They're set on traveling despite the storm warnings.