Salty water conducts. Oil, gas, and very tight rock do not. A deep-reading resistivity, together with porosity, is the usual input to a water-saturation calculation. The unit is the ohm-metre.
Mud filtrate invades the permeable rock, so a shallow reading and a deep reading can disagree. With water-based mud in an oil zone the filtrate is more conductive than the formation fluid, and the shallow resistivity falls below the deep one. That separation is a hydrocarbon indicator only when the mud and the formation water justify it. A resistivity with no porosity next to it cannot be turned into a saturation.