In layman's terms, Ground Penetrating Radar (GPR) works on the same principle as sonar or aerial radar (what the police use). A radar wave is pulsed into the substrate (ground, concrete, etc.) at a certain frequency.
Over time and distance, an image can be produced based upon the reflection of what the wave bounced off of. Typically, this image is in the form of peaks and hyperbolas in the data collected. If the wave finds nothing to reflect off of, it will gradually dissipate as it passes through the material. How deep the GPR wave travels is largely dependent on the material/soil it is trying to pass through.
Reflections are also produced when the electric conductivity of the soil changes. This electric conductivity is known as the dielectric constant of the soil and can affect how GPR works. Soil types that vary considerably with their dielectric constant show greater contrast than soil types that are closer in their dielectric number. The dielectric number is also a limiting factor in GPR penetration. Essentially, the higher the dielectric number, the lower the depth of penetration of the GPR wave.