A PTR record resolves an IP address to a hostname. This is primarily an RDNS thing. In fact, it's precisely that.
These are used for looking up an IP address and determing what exactly it is.
For example, try to do an nslookup on 127.0.0.1 . The hostname is going to return as localhost. This answer is supplied by the PTR record.
A bit better explanation from DNS Made Easy