On the picture, you see the move that was offered by the analyzer. My move was Na5. Forking the Queen and the bishop, so I can negate white's attack on f7 and defending the undefended pawn b7, so the queen can not take it. However, the analyzer labels it as a blunder and offers Nd4. Why it's better? Why is my turn a blunder, where can I lose here material?
1...Na5 isn't a losing blunder, Black definitely has a much better position after 1...Na5 2.Qc3 Nxc4 3.Qxc4 Nxd5. Black's lead in development and White's weak position (e.g. the g3 square and the king on e1) should be decisive.
However, 1...Nd4 is a even stronger move because the c2 square is under attack and can't be defended. White can resign immediately after 1...Nd4, whereas White could resist a bit longer after 1...Na5.
You had a winning position, but you weren't able to find the most decisive killing move. Whether this was a "blunder" is debatable.
Your move 1...Na5 is a blunder because there was an opportunity to win material.
The suggestion 1...Nd4 is best because you win a rook, e.g. 1...Nd4 2.Qxb7 Nxc2+ 3.Kf1 Nxa1
In addition to other answers, applications consider a move blunder when there is a much better move. By much, I mean like 150-200 centipawns. (100 centipawn equals to 1 pawn)
A blunder doesn't mean you're worse. If you're white and you play a move that goes from +10 to +7, then it will classify that move as a blunder. Same for if you're +3 and your move makes you +1.5; it will call it a blunder.
Na5 only wins a bishop (and a better position) for a knight and Nd4 wins a pawn with a K+R fork by attacking the queen.
