Please explain what this means

General
7

Did you also configure the Pi-hole IP in the router WAN/Internet DNS settings?
If so, you closed a partial DNS loop causing DNS queries to bounce back and fort endlessly until rate-limiting kicks in or resources are depleted.

Pi-hole --[CF]--> Router --
   ^                       |
   |                       |
    -----------------------

Check below for the recommended method to advertise the Pi-hole IP via the routers DHCP service:

Or if the router doesnt allow you to set DHCP service DNS, you can try below: