compare how an ammeter and a voltmeter are connected in a circuit
Voltage is the difference in potential between two points. So, if you connected a voltmeter in series all you are doing is measuring the difference in potential across a strand of wire which will be zero (or very near to zero). To measure voltage you connect the voltmeter in parallel so that you measure the voltage from the negative to the positive or the voltage drop across a load. Also, voltmeter has high value of resistance causing potential drop across the circuit which is not required. Amperes are the measure of how much electricity is actually moving through a wire. So to properly measure you must connect in series so that the actual current flows through the meter and proper value of current are measured.
Voltmeter - connected in parallel
Ammeter - connected in series