There are two modes in which your meter will be most useful - measuring resistance and voltage - so it would be good to learn how to use those. You might be able to find on line a manual for your meter that will help explain these settings. In these modes, it is possible to test different parts of your circuit quickly to find faults and loose connections. (Measurement of current (amps) is less informative in your setting and more difficult to do, so there are settings on your meter that you won't be using to start.)
Measuring resistance (which may require fresh batteries in your meter) will tell you if a circuit is in continuity. Whether the ground really connected to ground, for example, or how well things are connected.
Measuring voltage will tell you if you are delivering 120 volts on the correct lead, or (to take Kurt's example) if you have a voltage drop because of a long wire run or some other loss. Shorts can leak current creating shock and fire hazards.
Any measurements made with circuits energized must be made with care!
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