So I had this general impression that three-digit domain names were all taken, but it turns out plenty are available if you're okay with numbers in the URL. So of course I grabbed this one. The K is easy to pronounce and hard to get mixed up with other letters, and 55 is a nice repeated number, and .io is probably one of the best-known TLDs.
So yeah, this is just a domain whose subdomains are for me and my friends to host projects we're not ready to buy domains for. If you know someone who runs a .k55.io subdomain, and you want one for yourself, just ask them!