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 the rook is left unprotected after the move, exposing it to capture and weakening the position significantly. This kind of oversight can be costly, especially in high-level play where precision is key.

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

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 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

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?