Digital DC voltmeter. Measuring Voltage range is 0-100v. Voltmeter has three wires. RED, Black, Yellow. Here RED and BLACK is for Powering the meter. Power voltage should be up to 100v. yellow is measuring voltage  line. Connect the line on which you want to measure the voltage. if measureing voltage is woithin 30v then you can connect yellow and red on positive of the power line. But if its more than 30v then you need to give a seperate power of 3.7 or 12v to the meter otherwise the meter will damage.
Simply if you want to measure the voltage of a battery connect yellow & red in battery positive and black in battery negative.