The antenna of the radar level transmitter transmits a very narrow microwave pulse propagating at the speed of light in space.
When you encounter the dielectric surface to be measured, part of its energy is reflected and received by the same antenna.
The time interval of the transmit and receive pulses is proportional to the distance between the antenna and the dielectric surface to be measured.
Therefore, the distance between the antenna and the dielectric surface to be measured can be calculated.