The Burp Suite Collaborator is a valuable tool for penetration testers and bug bounty hunters. It basically gives you unique subdomains and logs all interactions (DNS, HTTP(S), SMTP) towards the subdomain. This can be used for example to detect SSRF-vulnerabilities and exfiltrate data.
Burp Suite Professional provides a collaborator service under the domain burpcollaborator.net and using it is usually fine. However on the rare occasions it can be blacklisted / blocked or otherwise unreachable from the target. Luckily, the Burp collaborator can also be self-hosted and set to use a whole custom domain.