You need to put a load on the battery to see if it has any charge left.
Without a load, it may show an acceptable voltage, but when you actually try to use it the voltage drops because the battery is nearly dead.
So to see if a battery is really usable you must measure the voltage when the battery is connected to a load.
Dead Battery, no load, 1.4 Volts
Dead Battery, load of 100 Ohms, 1.0 Volts
Good Battery no load, 1.5 Volts
Good Battery, load of 100 Ohms, 1.4 Volts
Those numbers are just representative - do NOT use them to actually measure your batteries. Check the unloaded voltage of a good battery, then check the voltage of a good battery under a typical load. Use that typical load to test other batteries. That is to say, figure out the equivalent resistance for the load and use a resistor of that value in your test.