The in-addr.arpa domain is a conceptual part of the DNS namespace that uses IP addresses for its leaves, rather than domain names. It is the part of your zone that enables address-to-name mapping.
Just as DNS domain names are read with the lowest level subdomain occupying the furthest left position and the root at the far right, in-addr.arpa domain IP addresses are read from lowest level to the root. Thus, the IP addresses are read backward. For example, suppose a host has the IP address 18.104.22.168. In the in-addr.arpa zone files, its address is listed as 22.214.171.124.in-addr.arpa. with the dot at the end indicating the root of the in-addr.arpa domain.