Again this word means that it says, that the coin in bright and shiny with its original mint lustre. It is possible for coins which have not been circulated to have lost this lustre through less than perfect storage or handling. Most metal will tone or tarnish over a period of time, often due to atmospheric chemicals, although moisture undoubtedly speeds this process.
Brilliant Uncirculated
Means that the coin described is in mint(although not necessarily perfect) condition with its original mint lustre.