Measuring the voltages of your batteries or any other connection is very important to debug issues. You can find out whether a connection has power or not, whether the connection is good, or the voltage is lower than it should be. A multimeter is a tool used to measure the voltage, ampere, and resistance:

A multimeter has one red and one black wire. The red wire must always be connected to the positive wire and the black wire to the ground or negative wire. For example, you can measure your battery's remaining voltage by connecting the appropriate wires, as shown in the following image:

You can use a multimeter to measure the voltage of any connection in your build. For example, if your LED strip does not light up, you have to check whether the connection that goes into the LED has power or not, as shown in the following image:

A simple and easy way is to find the two wires (black and red) that go to the LEDs and measure the voltage.

You will then be able to know whether the...