The value of material is obvious. The player with more pieces can generally do more with them. But this is not always the case. Having your pieces in the wrong places can be just as bad as not having those pieces at all. Sometimes trading off a more valuable piece for a less valuable one is the better choice. Whether it is to put your opponent in an extremely disadvantaged position, or to get yourself out of a bad spot,
Positioning your pieces properly is very important. When they are in the right places, pieces will show their true value, and will allow you to win more material and more games. Most pieces are more effective from the center, because from there they can control more area of the board, and can be mobilized more quickly from there. For this reason, and especially in the beginning and middle game, controlling the 4 squares in the center of the board gives you a big advantage.
Each move, when you look at what pieces can take what other pieces, and what can go where, always look at the position it will create. Sometimes you can disable your opponent's pieces by bringing them too far away from where they will need to be. Sometimes your opponent has a piece that is in such good position that it may be better to sacrifice one of your more valuable pieces for it to end the threat and buy you some time. This knowledge is gained by paying attention to the position during all of your games, and with experience you will start to understand how much value there is in any position.
Also, don't immediately write off a move just because it results in of a loss in material. If your remaining pieces are in good positions and enough in number, you may be able to use that advantage to either checkmate your opponent or take back as much or even more of his material.