Hi Chandu,
After the TX pulse you start the RX mode acquisition. At the begining of the acquisition you have a strong echoes from near located objects and in the time domain this echoes are weaker and weaker because the acoustic signal attenuates exponentially with depth. In order to equalize amplification over the penetration depth (time), you need to attenuate the signal more in the near field and leave it less attenuated in the far field (Time_Gain_Control). Hope it's clear.
Regards..