It is a material with infinite resistance (usually more than 1010 ohms). It protects the circuit by preventing the isolating components and conductors from touching each other, hence avoiding the possibility of a short circuit.

material that does not allow electricity to flow through it. (see conductor)

a material such as glass or porcelain with negligible electrical or thermal conductivity

Nonconductors or electrical insulators are materials which lack movable electric charges, and which therefore lack a low-resistance path for charge flow. When a difference in electrical potential is placed across a nonconductor, no free charges are exposed to the electric field, so no flow of charges appears, and an electric current cannot arise. Ideally, a nonconductor has infinite resistance and zero conductance.