Unit Impulse Function

This will become apparent as you learn more about convolution, which I must admit, I'm a little rusty on. From what I remember about the sifting property is thus:

Remember that the unit impulse function is equal to 0 every but at 0 in which case it is infinity.

The sifting property is seen when you integrate from negative infinity to positive infinity some function "F" times the unit impulse function. Because of the properties of the unit impulse function this just simply produces the value of F(0). If you were to shift the impulse function then evaluate the integral the answer would just be F(N) where N is the value that the impulse function was shifted by.

So in short the impulse function is used to "mask" a signal.