A series circuit is an electrical circuit where all components are connected in a single path which means that the current flows through each component one after the other, creating a daisy chain effect.
All components are connected in a single loop, so the current has only one path to flow through. The same amount of current flows through each component in the circuit. If any component in the circuit fails or is disconnected, the entire circuit will cease to function.
The total resistance of the circuit is the sum of the resistances of all individual components. The voltage across each component is proportional to its resistance. The lights are connected in series, so if one bulb burns out, the entire string will go out.