Very interesting question! Why when the Knight was on a4 the engine evaluation was -1 and when the Knight is on c5 the engine evaluation is -5? When the Knight was on a4 Black had an extra pawn, consistent with the engine evaluation of -1. Now that the White Knight is on c5, Black can attack it playing Qe7 followed by Ne4 (attacking the White Queen) and the threat of Qb4 check. So, Black has the opportunity to win some extra tempos and to put the Queen and the Knight in excellent squares.
White can avoid a check mate with a precise play, but even if that occurs, Stockfish "thinks" that that the endgame is hopeless for White. However, you need to take into consideration that an endgame could be hopeless for an engine, but not for a human. So, -5 is the engine "perspective," not a human perspective. For example, "my human evaluation would be -2" because Black has an extra pawn, better development, the threat of Ne4 and Qb4 check, and it will be difficult for White to castle. However, many good moves will still be required to win the game. The probability of making mistake exists. We all have seen or played games where Black is winning and suddenly White is winning because Black made a mistake.
Many years ago, a chess engine evaluation of +1 meant that White was winning with a pawn. For example, in the position that you are asking, many years ago, the engine evaluation would have been -1 because Black has an extra pawn. However, with the availability of $200 computers or $100 cell phones that provide powerful hardware resources, and the creation of modern chess software, chess engines can "see" in seconds 20 half-moves ahead in seconds (10 moves). Also, after those 10 moves chess engines can "guess" black and white chances. So, the engines calification -5 implies that most likely Black will win the game with a high probability (90-100%). If you give the hardware more time, they can "see" 50 and even 60 half-moves (between 25-30 moves).
When you post this kind of question, the best is that you post either all the moves of the game or the FEN that is a code that allows everybody to paste the position in a chess GUI. In this case, this is the FEN: r2qk2r/ppp2ppp/2n1bn2/2N5/2Bp4/1P2P3/P2Q1PPP/R3K1NR b - - 0 1
P.S. A half-move is a move of White or Black. A move is a a move of White AND Black.
P.S.#2 If somebody wants to play the position against the best level of Stockfish on Lichess using Black pieces this is the link: https://lichess.org/GQ9TqTKa/black
P.S.#3 If somebody wants to play the position against the best level of Stockfish on Lichess using White pieces this is the link:
https://lichess.org/84aSHZKU
