Resistance is the measure of how much the flow of current in a circuit is impeded. Copper has low resistance and so electricity flows through it easily. Glass and rubber have very high resistance and so they are not a good conductor of current.
The test described below should be done when current is NOT present. Always unplug the device or turn off the main circuit breaker before attempting this test.
Various devices rely on resistance to function, a heating element for example. Because the current faces resistance in a heating element, some of its energy is converted to heat. Resistance is necessary for heat to be generated in heating elements like those used in an electric stove or oven, dishwasher or hair dryer.
When testing for resistance, you will in some cases need to know what the proper resistance should be in order for the test results to be meaningful. However, "autoranging" multitesters or multimeters will adjust to display the proper resistance information.
Set the multitester to the ohm setting. If the meter is not an autoranging model, choose the ohm setting closest to the expected resistance of the device being tested. Touch one probe to one of the terminals and the second probe to the other, it doesn't matter which probe is used on which. The reading on the multitester should change from infinity to the level of resistance detected in the thing being tested. Compare the measurement to the manufacturer's specification for the device you are testing.