rbndr is a very simple, non-conforming, name server for testing software against DNS rebinding vulnerabilities. The server responds to queries by randomly selecting one of the addresses specified in the hostname and returning it as the answer with a very low ttl.

https://en.wikipedia.org/wiki/DNS_rebinding

DNS rebinding is a form of TOCTOU (time of check, time of use) vulnerability. You would use it if you have a service that uses "preflight" checks incorrectly to modify security properties. For example, consider a (fictional) browser plugin that has an api like this:

AllowUntrustedAccess("foobar.com");
SendArbitraryRequests("foobar.com");

And AllowUntrustedAccess() simply sends a preflight HTTP request to the host:

GET /CanIDisableSecurity HTTP/1.1

If the service returns 200, then the plugin allows the hostpage complete access to that hostname. This might be a security vulnerability, because you can specify a rbndr hostname that will switch between a host you control and a host you don't. The plugin might allow complete access to an arbitrary ip address (e.g. an internal service, or localhost) even if that service would not normally permit the preflight check.
This might sound unrealistic, but that's exactly how Adobe Flash, Oracle Java and lots of other products worked in the past, and many other products still work.
Read about how Adobe tried to resolve this problem in Flash here, https://www.adobe.com/devnet/flashplayer/articles/fplayer9_security.html
For software that is vulnerable to this class of attack, rbndr is an easy way to test without having to modify /etc/hosts or setup your own nameserver. If the software associates the result with just the hostname and not the hostname and ip address, then you can grant yourself access to any ip address.
The format for hostnames is simply

<ipv4 in base-16>.<ipv4 in base-16>.rbndr.us

But you can use this website to convert from dotted quads if you prefer:
https://lock.cmpxchg8b.com/rebinder.html
For example, to switch between 127.0.0.1 and 192.168.0.1 you would encode them as dwords, and then use:

7f000001.c0a80001.rbndr.us

Let's test it out:

$ host 7f000001.c0a80001.rbndr.us
7f000001.c0a80001.rbndr.us has address 192.168.0.1
$ host 7f000001.c0a80001.rbndr.us
7f000001.c0a80001.rbndr.us has address 192.168.0.1
$ host 7f000001.c0a80001.rbndr.us
7f000001.c0a80001.rbndr.us has address 192.168.0.1
$ host 7f000001.c0a80001.rbndr.us
7f000001.c0a80001.rbndr.us has address 127.0.0.1
$ host 7f000001.c0a80001.rbndr.us
7f000001.c0a80001.rbndr.us has address 127.0.0.1
$ host 7f000001.c0a80001.rbndr.us
7f000001.c0a80001.rbndr.us has address 192.168.0.1
$ host 7f000001.c0a80001.rbndr.us
7f000001.c0a80001.rbndr.us has address 127.0.0.1
$ host 7f000001.c0a80001.rbndr.us
7f000001.c0a80001.rbndr.us has address 127.0.0.1
$ host 7f000001.c0a80001.rbndr.us
7f000001.c0a80001.rbndr.us has address 192.168.0.1

As you can see, the server randomly returns one of the addresses. You might do something like this (in pseudo-code):

// Keep calling api until it resolves to the address you control and you get granted access
while (AllowUntrustedAccesss("7f000001.c0a80001.rbndr.us") != true)
  ;

// Access granted, now wait for it to re-bind
while (ConnectToPort("7f000001.c0a80001.rbndr.us", 123) != true)
 ;
 
 // Now you have access to localhost:123 even though localhost did not opt-in to reduced security.
 SomethingEvil();