Simply, the change in FREQUENCY when some noise is moving towards us or away from us. This process is known as the Doppler Effect.
In this concept, we everytime confuse whether we have to add velocities or subtract them.
But this is simple to understand by a minor concept of numerator and denominator.
As like when we have to increase the value, we either add value to numerator or subtract value from denominator. In both cases, the overall value will increase.
Similarly, for decreasing the overall value we either subtract from numerator or add in denominator.
The same case is for velocities, when the noise is coming which implies the sound increases means add velocity to numerator and subtract velocity from denominator.
And, when noise is moving away which implies sound decreases means subtract velocity from numerator and add velocity in denominator.
Hence, this is the candid concept of Doppler Effect which makes it easier to understand.