Schwann cells are the main Glia cells in the peripheral nervous system (PNS). Their main role is to provide support to the PNS, in particular, they are the principal component of myelination in axons. The Schwann cells form the myelin sheath by wrapping around the axon of the neurone, building up layers of insulation. One Schwann cell covers a specific segment in the myelin sheath i.e only one Scwann cell covers a specific axon. Areas without Schwann cells, and hence areas without myelination, are called the Nodes of Ranvier.
The myelination effect provided by Schwann cells greatly increase the conductivity of the axon and provide insulation to decrease current loss. In fact, a myelinated axon provides about 100 times the conductivity than an unmyelinated axon.