series circuit

A series circuit has a single path, so the same current passes through each component in turn. If any part fails, the circuit is broken and nothing works — the classic string of holiday lights that goes dark when one bulb dies.

The supply voltage is shared. Two identical lamps wired in series on one supply each receive half the voltage, so both are dimmer than either would be alone. The resistances simply add up, and because the current is the same everywhere, adding another component raises the total resistance, so the current falls and every lamp dims a little more.

Compare it with a parallel circuit, where several paths mean the same voltage sits across every branch and each branch works independently — which is why homes are wired in parallel. In the wording of a question, one path or the same current everywhere means series; several paths or the same voltage across each means parallel.