Voltmeters are placed in parallel with the components when the potential difference across a component is to be measured.Voltages measure energy changes as charge passes through a component.There will be a difference in energy from one side of the component to the other as energy is transferred by the charge to the component.Voltmeters should have very high resistances so that negligible current flows through the voltmeter.
Around any closed loop in a circuit, the sum of the emfs is equal to the sum of the potential differences. This is a consequence of the conservation of energy.