a conductor
A conductor is a material that lets electric charge move through it easily, the open highway for current. Metals are the classic example: copper, aluminium, gold, silver. The reason is that in a metal a sea of electrons is only loosely held and free to drift, so the slightest push (voltage) sets them flowing. This is why wires, contacts and circuit-board traces are made of copper: you want charge to get where it is going with as little hindrance as possible.
A perfect conductor would have zero resistance, but real conductors have a small resistance that turns a little energy into heat. Thicker and shorter wires conduct better (lower resistance); thin, long ones resist more and can warm up or drop voltage. Saltwater, the human body, and damp ground also conduct, just far more weakly than metal, which is exactly why electricity can be dangerous: you are a (poor) conductor too.
Knowing what conducts and what does not is the first thing you reason about when wiring anything. A short circuit is an accidental conductor where you did not want one. Even good conductors are not perfect: at high currents or long runs their small resistance matters, causing voltage drop and heat, which is why power lines use fat cables and high voltage to keep current, and therefore loss, down.
A 1 metre length of thin hook-up wire might have around 0.05 ohm; at 2 A that wastes only I^2 times R = 4 times 0.05 = 0.2 W, but a much thinner wire at 10 A could get dangerously hot.
Even a perfect-looking wire has a little resistance that matters at high current.
There is no sharp wall between conductor and insulator; it is a spectrum. A conductor simply sits at the easy-flow end, and even copper's resistance rises as it heats up.