I'm not familiar with it either, but from memory when I first installed Avast and Windows Defender is disabled, you also get an option to have it do periodic on-demand scans, I just wonder if this is what the Windows Security Service is.
That said Windows Security Service is on my Task Manager screen it is there (under Windows Security Health Service), there is also another process, Windows Security notification icon, but the resource use is negligible.
So I would generally follow the advice given not to disable it. I have never found there to be any sort of conflict or excess resource use.