They may not be perfect. Materials like copper, silver, and gold have very low resistances. In fact all metals will conduct electricity. They are called conductors. Even so, they do have some resistance to the flow of electrons through them.
Resistance is measured in Ohms.
Ohm's Law says that there is a relationship between these three factors. So if you know two of the values you can easily work out the third one. If you know the current and resistance and want to calculate the voltage, you use the first equation. If you know the voltage and resistance and want to calculate the current, you use the second equation. Lastly, if you know the voltage and current and want to calculate the resistance, you use the third equation.