The strategic sacrifice is a strategy of getting out of losing a primary piece to your opponent. At times your opponent is going to be attacking one of your primary pieces and that primary piece of yours is unprotected. If you have no way for you to protect that primary piece against the ensuing attack, or no option to retreat, then a strategic sacrifice may be in order. An inexperienced player may simply allow the threatened primary piece to be captured and move on, but this does not have to be the case.
When you find yourself in this situation you may be able to threaten one of your opponents primary pieces on the very next move. If you are able to do this it is a good idea since you cannot retreat and cannot move a piece to protect your threatened piece. For example, you can slide your bishop up to attack his unprotected rook. This places your opponent in a difficult situation. Your opponent must decide if he wants to save his primary piece that you are threatening, or go ahead and take your primary piece that he has threatened. Of course, he may decide to take your primary piece anyway, at which point you take his primary piece. This results in a compromised loss for you in that you at least gain one of his pieces instead of none at all. If you moved correctly, perhaps your bishop is now only one move away from protecting your original piece assuming your opponent decided to protect his threatened piece. This is a good way to keep the pieces on the board more even if you do find yourself in this type of situation during a game.