A Developer Advocate — a.k.a. Developer Evangelist — is someone who works in Developer Relations (DevRel for short) and enables developers to use a product or technology. In a way, a Developer Advocate is like an extremely knowledgeable influencer who makes educational content, like videos and blogs, fostering relationships with developers that use specific software technologies and/or services.
For more information on the role of a Developer Advocate see the article here