I need a dumbed down version of this.
replies(7):
There are certain circumstances where private network operators may wish to use their own domain naming scheme that is not intended to be used or accessible by the global domain name system (DNS), such as within closed corporate or home networks.
The "internal" top-level domain is reserved to provide this purpose in the DNS. Such domains will not resolve in the global DNS, but can be configured within closed networks as the network operator sees fit.
This reservation is intended for a similar purpose that private-use IP address ranges that are set aside (e.g. [RFC1918]).