"Discrimination" is in some way treating people differently from other people. Much discrimination is a sound idea. Not letting someone fly a plane who does not have pilot training is discrimination, but not a bad kind. So is not allowing someone to do surgery who has not trained as a doctor.
Unlawful discrimination is discrimination that a particular law in a particular country declares to be forbidden. Different countries have different laws for what kinds of discrimination is unlawful under what circumstances.
Discrimination on a basis not forbidden by law is legal, even if immoral. For example, a restaurant could, in most if not all countries, ban left-handed people from being served there. This might be immoral, but would not be illegal, because no law has made it illegal.
Commonly, discrimination on the basis of religion, ethnicity, national origin, sex, or political opinion is unlawful. In some jurisdictions discrimination on the basis of disability is unlawful. Other bases may be unlawful in some places. There may be exceptions. Employment discrimination may not have the same standards as housing discrimination, and admission to public places or ability to run for public office may be different yet. All this will vary based on the particular laws of particular places.
Note that some of these protected classes are things about which people have no choice, such as sex and national origin, others are matters of choice such as religion and political opinion. Laws may create a protected class on any basis. Note also that many things about which people have little or no choice, such as height, handedness, or intelligence, are not protected categories in most places.
To the best of my understanding being vaccinated or not is not a protected class in most if not all of Europe, so legal distinctions may be made based on vaccination status. In such jurisdictions, discrimination in favor of those who are vaccinated is legal, although some may think it wrong.
A few US states have made it unlawful to restrict various public or private services or access to places on the basis of vaccination status. I am not aware of any country in Europe that has such a law.