If free electrons are plenty in a substance, the amount of current is more for same applied voltage across the substances. The current through a substance not only depends on the numbers of free electrons in it, but also depends on the length of path an electron has to travel to reach from lower potential end to higher potential end of the substance. In addition to that, every electron has to collide randomly with other atoms and electrons in numbers of times during its traveling. So, every substance has a property to resist current through it and this property is known as electric resistance.
When, we apply a voltage across a substance, electrons start drifting from lower potential end to higher potential end of the substance. If we increase the applied voltage, the drifting of electrons in the substance also get increased accordingly. The electrons flowing from negative to positive end of the substance, means, flowing of negative charge from lower potential end to higher potential end. The rate of flow of charge is current. Hence, we can say, current through a substance is proportional to applied voltage to the substance. This relation is linear for metallic substance. This relation between current and voltage holds its linearity as long as the temperature of the substance is constant. If temperature changes, the inter atomic vibration in the metallic substance changes. This effects the free paths for travelling electrons. Hence, for fixed applied voltage, if temperature changes, the resistance of the metallic substance is also changed. So, if we change both applied voltage and the temperature of the substance simultaneously, the relation between current and voltage remains no more linear. But at constant temperature, the current is directly proportional to applied voltage. We denote here, the constant of proportionality, with R, and this R is known as resistance of that metallic substance (conductor). If one volt across a conductor produces one ampere of current through it, then the resistance of the conductor is said to be one ohm (Ω).