The pressure gradient informs us of the pressure difference between the left ventricle (LV) and aorta during systole. From the velocity-time interval (VTI) tracing of the AS jet velocity, the machine calculates the peak/maximum and mean pressure gradients.
https://www.cardioserv.net/echo-top-5-factors-to-consider-ev...

Doppler echocardiography enables noninvasive determination of blood velocity and flow area through which quantitation of blood flow in vessels and across valvular orifices can be achieved

https://www.researchgate.net/publication/19379397_Determinat...