Burp Collaborator is a service that is used by Burp Suite when testing web applications for security vulnerabilities. Some of Burp Suite's tests may cause the application being tested to interact with the Burp Collaborator server, to enable Burp Suite to detect various security vulnerabilities.
The Burp Collaborator server does not itself initiate any interactions with any system, and only responds to interactions that it receives from other systems.
If you are a systems administrator and you are seeing interactions with the Burp Collaborator server in your logs, then it is likely that someone is testing your web application using Burp Suite. If you are trying to identify the person responsible for this testing, you should review your web server or applications logs for the time at which these interactions were initiated by your systems.
For further details about Burp Collaborator, please see the full documentation.