Electricity flows through some substances more easily than others.
By measuring how well electricity flows through underground rock formations engineers can better understand what's down there. In the experiment we use an ohmmeter to measure electrical resistance of dry soil and compare it to the resistance of soil soaked in salt water and then with oil soaked soil. The dry soil will probably have a very high resistance, probably so high that it will not register on the ohmmeter at all. With salt water there will most likely be a measurable resistance. With oil instead of water the resistance should be very high.