Externel IP address means the IP address on the public internet side. Commonly our PC might has the internal IP address range which is assigned from gateway such as NAT(Network address translation).
NAT help us to use one public internet IP over many network devices.
The reason that you have to check the external IP address is that it is needed to connect the NVR or camera from the internet. The client uses the external IP to reach the target.
You can check the external IP address if you visit the following sites.