Why is this a Blunder?

i did this skewer, with the queen and rook, but it is a blunder? Bc8 to g4

It’s a blunder because you went from being in a single attack with your knight to a double attack with your bishop had your opponent played the correct move f3. You got away with it because your opponent blundered and you were up material but the stronger you get the less forgiving stuff like this goes.

It’s a blunder because the rook is left unprotected after.

no, because there is nothing atacking the rook

You will either lose a knight or a bishop which loses material.

but they will loose a queen or a rook

Like I explained in my earlier comment, he missed the correct F3 move. Stronger players will see this and then you will be in a double attack

yes i agree with dunnbrotha -- please explain the losses of material in concession if a blunder is made - I don't see the blunder and if it was a blunder what move that gains material or advantage should they have taken on that move ????

If f3 all of a sudden you have both your bishop and knight under attack and are losing a piece. However what i will say is you need to evaluate yourself, dont just skip through the "Game Review" as that isnt a proper way to learn anything, for instance right now you have 0 clue what you did wrong. In the top right there will be a little magnifying glass you can click to look at the engine review in a detailed way. Make sure to set up your settings to show multiple lines etc.

i don't know

If they block with the pawn, you have to move the bishop back, and then they will have the upper hand because of the double attack on your under defended Knight

Edit: f3, Bh5, Bxe5

I wasn't very wise to post a screenshot instead of a link to the game. What good is a screenshot for?

People have been given the right answer (except for ColetteBrady who probably posted in the wrong topic). White plays f3 and black loses a piece, as it's impossible to save both the e5 knight and the g4 bishop.

Also, it's astonishing how useless and bad Game review is. "This move loses a knight" is not correct and not helpful. The way to go is to get out of Game review and go to Analysis, which is a small magnifying glass icon either on the top or the bottom of the screen.

idk

Look at the position. Black already has a large advantage due to being up a piece. After 1...Bg4 2.f3, white has to deal with the threat to the bishop and the on the e5 knight thus resulting in a position where white will be a pawn up at best.

Instead of hanging a piece, play a move like 1...Ng6

The best way to understand material loss or gain quickly is by counting how many pieces control that square. Your knight on e5 is being attacked by two pieces, the queen and the bishop. You only have one defender, your bishop. That means you will lose material. So your best move is knight to g6 to attack his bishop. Now he has 2 pieces attacking it, and no defender. This leaves him an unfavorable choice of having to trade off his active dark square bishop, because retreating means both your bishops and queen are pointed at his king with mate threats. 
Instead you self inflicted a double attack by attacking the f3 square which is controlled by 3 pieces, his pawn, rook and queen, and you only had 2, the bishop and knight. More material loss. You are only looking at the attacking on his queen and rook but you have to see the squares outside the ones you are attacking, especially if they have a counterattack, which they did, with f3. 

Finally. I thought I was the only one.

I once went through a game review where my opponent allowed me to win his bishop by trading queens and the coach said "this loses a knight." The analysis and follow-up even showed that the correct sequence of moves, which I played, wins a bishop instead of a knight.

And of course, there's the occasion where it gives useless information for a terrible blunder. For example, I've also seen: "This gives away a free pawn. This allows the opponent to play checkmate." Like, why does the first part even matter?