Why is a voltmeter always connected in parallel and what will happen if it is connected in series?

A voltmeter is always connected in parallel so that it measures the potential difference across the component without significantly altering the circuit. If connected in series, it would drastically change the circuit behaviour and give incorrect reading (likely drop almost all voltage and stop normal operation).

• Why parallel: A voltmeter is designed to have a very high internal resistance. When connected in parallel with a component, almost all current flows through the component, the meter draws negligible current, so the circuit is not disturbed and you correctly measure voltage across that component.
• What if connected in series: If you connected the voltmeter in series, since it has very high resistance, almost no current would flow. The circuit would be effectively open, the component might not operate correctly, and the meter would measure the entire supply voltage (or near zero depending on circuit), thus reading is meaningless and circuit operation is disrupted.