Why is this Brilliant?

the move was Kb5

Well, because it is a decent sacrifice. That is basically all a "brilliant" move is here. According to the Chess.com Help pages, it should also be the best move or nearly the best and shouldn't make your position a lot worse. It didn't. (It is the 5th best move according to the engine.)

You are sacrificing your bishop by moving your knight away. The queen and knight are attacking it and it is only defended by the queen.

You could have moved the bishop away to a number or squares. But you are threatening a knight fork on c7, picking up the rook on a8.

You did see it was hanging, didn't you? The program is assuming that you saw the threat but still went ahead and left the bishop under-defended in order to get the knight fork. After everything, you lose a bishop but you win an exchange and a pawn, which is basically an okay trade.

You saw all that, right?

yes thank you

Kxc7 is mate

It's hard to guess what you're talking about. Maybe a different position?

It definitely isn't "brilliant" even according to chess.com's definitions.

However, it's best to teach people correct terminology. It isn't a sacrifice by any stretch of the imagination. It's properly called a "combination" and it certainly isn't brilliant because it makes white's position worse rather than better.

A combination is basically a series of moves of a tactical nature. In the initial position, white has slightly better development but then squanders it to trade two minor pieces for rook and pawn, which is helping black. It isn't quite an equal trade, since R+p are slightly less in value than B+N. But more to the point, the B and N are developed and active and the R is not. So the whole thing is a bad idea by white and pretending that it's "brilliant" is going to harm the chess of learners or beginners, rather than helping it.

Um actually the king can flee to d8

Or e7

Kd8?

In chess notation, K stands for king and N stands for knight.