Standard DNS (Forward DNS) maps a domain name to an IP address. Reverse DNS does exactly the opposite: it maps an IP address back to a hostname.
Reverse DNS lookups use a special type of DNS record called a PTR (Pointer) record. Unlike an 'A' record which sits under the domain name, a PTR record sits under a special reserved domain called in-addr.arpa.
Many mail servers will reject emails from an IP address that doesn't have a valid Reverse DNS record. It proves the sender isn't just a random hijacked home computer.
Instead of seeing a list of raw numbers like 172.217.1.1 in server logs, administrators see google.com, making it much easier to read.