PTR records are often called Reverse DNS records. A Pointer record (PTR) resolves an IP address to a fully-qualified domain name (FQDN) as an opposite to what A record does.
PTR records are mainly used to check if the server name is actually associated with the IP address from where the connection was initiated.
Here's how it's suppsed to work. Suppose your IP address is 126.96.36.199:
188.8.131.52 --- PTR Record ---> hostname.domain.com
hostname.domain.com --- A-Record ---> 184.108.40.206