Back in the early days of The Internet, when routers rode dinosaurs to work and nerds weren't cool, we wanted to signal to our network neighbours certain information about routes. To be fair, we still do. But, back then everyone had 16 bit ASNs, so there was a simple concept called 'communities'. This was a 32bit opaque value, that was traditionally split into two 16bit values. Conveniently, we were able to encode an "us" and a "them", and perform actions based on what our neighbours told us.
But, 16bits is pretty limiting. There could only be ~65'000 possible networks on The Internet total? Eeek. So, we created 32bit ASNs. 4 billion networks is seen as a quite reasonable limitation. However, you can't really encode a 32bit "us" and a 32bit "them" value into 32bits of total space. Something called "Extended Communities" was invented, but it tries to solve everything except the case of a 32bit ASN signalling to another 32bit ASN.
Enter Large Communities. This is 3 32bit values. The first one is the "owner" of the namespace. Normally, you would put in your own ASN, or the ASN that you wish to signal. The second two 32bit values are opaque and only have meaning from the originating operator, but normally people will use "myasn":"verb":"noun" Or "myasn":"noun":"verb". Either way, it fits very nicely.
Having previously ran a 32bit ASN, it became quickly obvious the lack of suitable communities was a critical problem. It was even the way to request an "old style" 16bit ASN from RIPE, "I need to use communities". Even the ability to say "do this to that ASN" was ugly, since you couldn't really communicate who the community was supposed to matter to. Clearly, we The Internet Community screwed up by not addressing this need earlier.
OpenBGPD in OpenBSD -current has support for Large Communities, and this will be available in the 6.1 release and later. This was based partially on a patch from Job Snijders, thanks!