Measuring resistance in electrical items is relatively simple with the right tools. Resistance is the measure of difficulty electrons have in flowing through an object. It is a little bit like friction when objects are moving over surfaces. Resistance is measured in ohms; 1 ohm is equal to 1 volt of electircal difference per 1 ampere of current. Resistance can be measured with an analog or increasingly with digial multimeters or ohmmeters.
When you are testing resistance, you should do so on components rather than in a circuit. Measuring resistance on a component when it is in a circuit will often cause inaccurate readings, because the multimeter is measuring the resistance from the other components in the circuit as well as the one being tested. Sometimes, it is useful to test within the circuit though.