Resolver IP is the DNS Server IP of website. It is an optional parameter for OCSP stapling. OCSP stapling itself is an optional parameter but it is highly recommended to use it as it will improve SSL handshake speed of website.
If resolver IP it is not added, it defaults to the server's DNS IP.
You can get Server's DNS IP using following commands in Linux
Command: cat /etc/resolv.conf
options timeout:2 attempts:5
; generated by /sbin/dhclient-script