This is a special domain used for reverse DNS lookups. In the in-addr.arpa domain, IP addresses are represented as a sequence of four decimal numbers separated by dots. The suffix of in-addr.arpa is appended to the IP address. In this domain, however, IP address octets are read from right to left, but the contents of the octets are not. For example, in order to do a reverse lookup on 172.217.169.174, you would use 174.169.217.172.in-addr.arpa. This domain is used for reverse lookups on IPv4 addresses.
For IPv6 addresses, the ip6.arpa domain is used. The address is represented by hex digits in reverse order - each digit looking like a domain name. For example, to do a reverse look up on 2001:db8::567:89ab, you would use b.a.9.8.7.6.5.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa.