Your eyes can’t show you what’s happening inside a circuit, but a meter can.
Every meter has a “common” socket, color-coded black and labeled “com.” Plug the black probe into this socket.
You’ll also find a red-coded socket, labeled with the letter V and an ohm symbol to tell you that it measures voltage and resistance. If it is also labeled mA, it will measure current in milliamps (mA) — but a separate socket may have that purpose. Yet another socket is set aside for high currents, up to 10 or 20 amps.
Connect your red probe to the appropriate socket and turn the selector dial to the units that you want to measure before you touch the probes anywhere.
Touch the black probe to the negative side of the power supply, touch the red probe to any other location in a circuit, and you’ll monitor the voltage (electrical pressure) between them. If you have an auto-ranging meter, it takes a moment to respond. A manual meter is faster, but you have to turn the dial to specify the upper limit of the voltage range.
Checking voltage is the most basic way to troubleshoot a circuit — you can look for bad connections, burned-out components, or a dead battery.
Current consists of a flow of electrons. Pass it through the meter cautiously, as too much current will blow the internal fuse. When trying to measure current, never attach the meter directly to a power source (as seen above).
To measure the resistance of a component, disconnect it from a circuit. The value in ohms tells you how strongly a component will resist current.
Most simple electronic circuits use DC power, but meters can measure AC too. Many have an AC-DC button, but some have AC options on the selection dial.
You can buy a meter for as little as $5 on eBay. Costlier meters will be more accurate, will do auto-ranging, will contain a replaceable internal fuse, and their switch contacts may last longer. They will also have extra features, such as measuring the value of a capacitor or the frequency of a stream of pulses from a microcontroller. They’ll also do continuity testing, emitting a beep when wires are properly connected or a switch is closed.
You’ll find that I discuss more aspects of meters in my book, Make: Electronics.