Ohm’s Law is based on the material’s conductivity. The opposite of conductivity is resistivity. If a material conducts well, then it has less resistance and vice versa. The resistance of any material is explained here first:

The power source is a generator which brings a coulomb up in potential. The coulomb has a joule of energy increased by the power source and is therefore valued at 1 volt. Voltage is potential energy per coulomb. So 1 volt is 1 joule of energy added to a coulomb.

Now the coulomb charge is to be fallen down and releasing the energy gained from the power source to the material that it flows through. Now it depends on the resistivity of the material. The time it took to flow through varies. Higher the resistance will take a longer time for the coulomb to flow through the material. Lower the resistance will take a shorter time of course.

In general, time is a measure of the resistance of the material. Resistivity is a measure based on the normalized geometry of the material. It can be obvious that lengthier the material, higher the resistance. This is why resistance symbol is zig-zag path. Increasing the area helps reducing the resistance. It is because the material with more area kind of allows the coulomb to flow in multiple channels.

Therefore we have R=r l/A formula to calculate resistance.

One can also see that increasing the potential, ie the energy of the coulomb, the time the coulomb takes to travel is reduced as it has more energy, faster. So V and R is proportional. Of course R and I is inversely proportional because I is Q/t. Increase t thus decreasing I. We just say that increasing R will increase t thus decreasing I, ie, inversely proportional.

Ohm’s law just stated that R=V/I. Of course R=r l/A means r = RA/l for any material’s resistivity measurement.