The main thread is the main render thread for DX11. The way DX11 works, there’s one thread that handles most tasks, with a few secondary threads. Once that main thread hits 100%, the secondary threads are limited and you effectively have a bottleneck. It’s like having 5 people performing work, with one person delegating the tasks.
This is largely a limitation of the DX11 rendering pipeline. Having a high single-core boosting frequency will help, but you’ll still be somewhat limited. Even on my Ryzen 5950x with a 5 Ghz single-core frequency, I’m largely limited by the main thread with LOD at 400. The “Limited by main thread” issue will likely be gone with the move to DX12, although it’s debatable if it’ll help performance.