Burp Suite collaborator is a service used to detect vulnerabilities mostly when an application tries to interact with external services. Burp Suite analyzes the interactions with external systems and detects unusual behaviors. In order to analyze the application, Burp Suite collaborator sends inputs or payloads to the application and waits for a response.

So, in this case, Burp Suite is working a server, where the application interacts using common services, such as DNS, SMTP, or HTTP.