Most meters apply a small DC current to the test leads that you connect to the component being tested. The meter can determine what the resistance of the component is by measuring the amount of voltage across the test leads.

R = E/I

Where:
R = resistance
E = voltage
I = current

Because the coils in pickups have a property called inductance, they display different values of impedance at different frequencies. Resistance is a DC-only characteristic - with AC, you have to deal with impedance, specified at a particular frequency. Since not all manufacturers specify the pickup impedance at the same frequencies, it becomes simpler to compare the DC resistance value. The coil is only one factor in determining what a pickup will sound like, but generally, a higher DC resistance indicates a larger coil, which usually gives higher output.