An ammeter has a very low resistance and should be used in series with the load, the resistance is vary small because it shouldn't alter the circuit operation by introducing voltage drop across the ammeter.
If you connect it in parallel with your voltage source it behaves like a short circuit and it can damage your meter and/or source.
A voltmeter on the other hand is designed to have a high resistance because it shouldn't add a load in the circuit you measure so that the voltage of the circuit doesn't change, if you use it in series then the circuit will not work, nothing will be damaged, it is like placing a 1M or 10M ohm in series with your circuit.
Alex