An onion link checker is a tool that verifies the accessibility of .onion websites, which can only be accessed through the Tor network. The post details how an online checker works, its use cases like avoiding fake links, monitoring uptime, and testing response times. It highlights Browserling, a service offering an onion link checker, which simplifies the process without needing a Tor browser installation. The content serves web developers and security analysts for fast and secure testing of onion links.