The receiver is the component in an ultrasound machine that receives each echo's Rf signal from the beamformer and modifies it before sending it on to Memory.
The primary functions of the receiver are:
Amplification
Compensation
Compression
Demodulation
Rejection
The sonographer can typically control four of these functions, so you should have a good understanding of the receiver.
Here are two videos that explain these 5 important functions of the receiver: