Ohm's law states that:
The ratio of the current in a conductor to the potential difference (voltage difference) between its ends is a constant as long as the temperature stays constant.
This constant is called the RESISTANCE of the conductor.
Resistance = Voltage (V)/Current(I) or R = V/I
Voltage = Current x Resistance or V = IR
Resistance is measured in units called Ohms ( W ).
The resistance of a piece of wire is 1 ohm if a current of 1 A flows through it when a voltage of 1 V is applied between its ends.
Using an ammeter and a voltmeter to measure resistance
To measure the resistance of say a piece of wire or a resistor we must find the voltage (potential difference) between its two ends and the current flowing through it.
An ammeter is always connected in series with the component and a voltmeter is always connected in parallel with the component