A series circuit links electrical items together so electricity flows from the power source and then through every item connected until it returns to the power source. It is effectively a single ring of wire. If you have several light bulbs wired into the circuit and one bulb fails, all the remaining lights turn off, so until you find the blown bulb, you don't have any lights. This is because once the filament in the bulb fails, the circuit is broken and electricity can't flow through it to complete the circuit.
Every item wired into a circuit, including the wire itself, creates resistance and resistance reduces the amount of power each item gets if it's a series circuit. For example, if your power source provides 12 volts and you wire four 12 volt bulbs in series, the amount of power each light bulb gets is 3 volts. This is because series wiring equalizes the voltage supplied to each bulb; it can't supply 12 volts to each bulb. The result is that the lights are 75 percent dimmer than if a single light was connected to the power source.
Wiring batteries in series increases output voltage, so to provide 12 volts to each bulb wired in series, you need to wire four 12 volt batteries in series. While this effectively sorts out the problem, it does translate into cost. To power four lights in series, you need to buy an extra three batteries.
A series circuit can be unreliable as there are many things that can go wrong if the circuit is compromised. You may have several batteries wired together resulting in many wired connections. The wire used to connect the various items in the circuit is connected in a single loop to all the terminals on the electrical devices, increasing the number of connection farther. Finally, every electrical item wired in the series circuit is reliant on the others continuing to operate. If one connection fails anywhere in the circuit electricity ceases to flow. If one electrical item fails, the circuit is broken and all the items fail.