ULA (Unique Local Addresses, or Unique Local IPv6 Unicast Addresses to give them their full name) are IPv6’s equivalent of IPv4’s “private” addresses.
The idea is to append a random 40 bits to the reserved ULA prefix fd00::/8, thus building a /48 that you can call your own. You can use this /48 wherever and however you like, with the sole proviso that it must not be routed on the public Internet. The ULA space is defined in RFC4192 (http://tools.ietf.org/html/rfc4193).