an insulator
An insulator is the opposite of a conductor: a material that strongly resists the flow of charge, the blocked road for current. Rubber, plastic, glass, dry wood, ceramic and air are everyday insulators. In these materials the electrons are tightly bound to their atoms and are not free to roam, so an ordinary voltage cannot get them moving. This is why wires are wrapped in plastic and why you can hold an insulated handle safely.
No insulator is perfect: push hard enough, with a high enough voltage, and even air will break down and let charge jump through, which is exactly what lightning and a spark are. The voltage an insulator can withstand before it gives way is its breakdown voltage, or dielectric strength. For hobby low-voltage work, ordinary plastic insulation is wildly more than enough; for mains and high voltage, insulation thickness and rating become a serious safety matter.
Insulators do the quiet, essential job of keeping current on the path you chose: the coloured plastic on jumper wires, the green solder mask on a circuit board, the gaps between traces. They also store energy in capacitors, where a thin insulator (the dielectric) sits between two conducting plates. Choosing the right insulator, its strength and its heat tolerance, is as much a design decision as choosing the conductor.
The thin plastic coating on a jumper wire might withstand 300 V before breaking down, which is why two insulated wires can safely cross and touch in a 9 V breadboard circuit.
Insulation is what lets wires touch without shorting.
Insulator is relative, not absolute; every insulator leaks a tiny current and will conduct if the voltage is high enough, so never trust ordinary plastic against mains-level voltages.