To estimate the Chess Pieces Value we take the pawn as a unit of measurement.
During a chess game you often trade in pieces for others. When you capture a piece it usually will be recaptured as the opponent is not so generous to give it to you for free. So you should know the value of the various pieces.
Just remember these values as a guideline. In general you want to capture as much material as possible. After that you do trade off so many pieces and pawns as possible to simplify the position and to win in the endgame.
The reason you win is because you have an overwhelming material advantage, which will win in the long run. When you have more material than your opponent, and the position is equal, you normally win sooner or later. It is just a matter of time, unless you make a big mistake.
So try to get his rook for your knight or bishop and start simplifying the game in trading off pieces. Trade off his queen if you can. This will have a strong simplifying effect on your game. Then open lines for your rook and penetrate into his position to kill his pawns from behind. You have a rook against his knight or bishop in the endgame so you should win because your rook will be stronger.
The difference in chess piece value between a rook and a bishop or between a rook and a knight, is 2 pawns if the position is equal.
Do exchange your knight or bishop for the rook of your opponent to gain a material advantage in an even position.
The value changes as the game goes on and depends upon the structure of the particular position.For example: