In Denmark some professions are regulated by law, e.g. lawyer, authorised psychologist, financial advisor, dentist, or doctor. If you want to work in Denmark within a regulated profession you must apply for a Danish authorisation or similar official approval. The conditions and rules depend on the Danish legislation for the individual profession.
You can see a list of the professions regulated by law on Ministry of Higher Education and Science’s website.
Here you can also read more about how to apply for an authorisation, i.e. how you get permission to work within a profession regulated by law.
If you have been trained as a medical doctor or dentist in another country than Denmark, you can apply for a residence permit in Denmark that makes it possible for you to learn Danish, pass the professional tests etc. necessary to obtain a Danish authorisation as a medical doctor or dentist.