In an electrical circuit, there are basic two ways in which components can be connected. One way is series connection.
In series connection, all the components are joined to each other like a chain in a single line style.
Current just has one path to flow from. Current flows from the source and then reaches one component and then other and so on. As a result, current flowing through each component is same.
If any component in the circuit is not connected properly, or broken, the current will have no path to flow. So in series connection, if one of the component is not connected or removed from the series, circuit will be incomplete and circuit will not function.
The voltage across the series circuit is the sum of voltage across each component.
The most common series circuit is the on/off switch connected to all the gadgets in our home. For example, the Fan in our room is connected in series with switch and power source. If the switch is off or broken, the fan will not work.