A multiplex network is a network model composed of a multiplicity of overlapping networks that capture the different types of connections between nodes within the network. When analyzing a large system like a metropolitan economy, a corporation or the global commodities market, what we are dealing with is a network that is embedded within many other networks. A metropolitan area will be a complex system where economic interactions are embedded within socio-political networks, transportation and geospatial networks and financial networks, in order to capture all of these overlapping networks it is optimal to use a multiplex network model. In a multiplex network, each type of interaction between the nodes is described by a single layer network and the different layers of networks describe the different modes of interaction.
Multiple networks are pretty complex but see this paper for an introduction: https://goo.gl/B1Oed0