Just so people have an idea what they are doing, the important things to know are.
Volts = Potential difference.
Amps = Current
Ohms = Resistance
For an example and ease of explanation, you've got a 12v battery with a bulb connected with a positive wire between the positive terminal of the battery to the bulb and a negative wire between the negative terminal of the battery and the bulb.
If you were to measure volts between the positive and negative of the bulb, you'd get a reading of 12v as there is a 12v difference between the positive and negative side. However, if you were to measure positive to positive, you'd get 0v as there is no difference.
For amps, you have to break into the circuit to measure, so you'd take away the negative cable and put the positive multimeter wire to the bulb and the negative multimeter wire to the battery, this allows you to measure the amount of current flowing through that circuit.
For ohms, if you measured volts across a wire or a component like in the first example and the voltage shown on the gauge is less than the system voltage, this is called a voltage drop and is caused by high resistance. To measure the resistance of a wire or component, you'd disconnect this wire or component and put the multimeter probes each end and this will tell you the resistance. A high reading on a wire would lead you to corrosion or a broken wire.