ACS712 is a Hall Effect-Based Linear Current Sensor it can measure both DC(Direct Current) and AC(Alternating Current). This sensor operates at 5V and produces an analog voltage output proportional to the measured current. This tool consists of a series of precision Hall sensors with copper lines
This sensor has an output voltage of Vcc x 0.5 = 2.5 at the input current 0A and a 5V Vcc power supply. There are three types based on the readable current range,
± 5A, ± 20A, and ± 30A
with output sensitivity of each type of 185mV / A, 100mV / A, and 66mV / A respectively.
The output of this current sensor is analog, so to read it, we can directly measure the output voltage  by using a microcontroller like Arduino through Analog Read pin or ADC pin.
 
IP+ & IP-
Connect the sensor in series to the system whose current you want to measure. Cut the circuit wire and connect one end of wire to IP+ and other to IP-. Recall that, current can be measured only in series. So don’t connect the sensor in parallel You may damage it by connecting it in parallel.   
VIout
Viout is the voltage out pin. ACS712 outputs an analog signal in correspond to any variation across the IP+ and IP- pins or in other words it outputs analog voltage on VIOUT pin if their is any variation in the current.