Anyone can act as an advocate, but it is most commonly used to describe a person who provides support and acts on behalf of another person or group, especially in a legal or political context. An advocate can be a professional, such as a lawyer, who represents a client in a court of law or before a government agency. An advocate can also be a non-professional, such as a family member or friend, who provides support and assistance to someone in need.
In a broader sense, an advocate can refer to anyone who publicly supports or campaigns for a particular cause or issue, such as an environmental advocate or a consumer advocate.