Nodes of Ranvier are microscopic gaps found within myelinated axons. Their function is to speed up propagation of action potentials along the axon via saltatory conduction. Thus, the Nodes of Ranvier are the gaps between the myelin insulation of Schwann cells which insulate the axon of neuron.
Related Theory:-
The nodes of Ranvier allow for ions to diffuse in and out of the neuron, propagating the electrical signal down the axon. Since the nodes are spaced out, they allow for saltatory conduction, where the signal rapidly jumps from node to node. This rapid rate of conduction is called saltatory conduction.