There are several sectors that constitute economic infrastructure. Some are real infrastructure like roads, the electric grid, but others are not typically seen as infrastructure. The housing market, for example, is not typically seen as infrastructure, but it is part of the economic infrastructure—a necessity to building economic prosperity.
Other examples of economic infrastructure are healthcare, education, and media. In order to build economy, people need health, they need a knowledge base, and they need to filter new information through that knowledge base to keep it healthy and current.
The importance of economic infrastructure is two-fold. First, it provides the same support role that traditional infrastructure provides: it girds the other social and economic activities of a society. It allows commerce to operate efficiently and with routine expectations that fade into the background of life, letting those engaged in other activities focus on their local problems and challenges. Second, just like traditional infrastructure, it creates a base of economic activity to itself. This base activity furnishes a minimum and continuous economy that can cushion the dynamic economy that sits atop it. Even when downturns occur, children continue to go to school, medical practices continue to operate, and housing is still needed for all inhabitants.
Those that argue, for example, for Medicaid expansion in the states, are arguing for improvements to the economic infrastructure. As with traditional infrastructure, more developed societies should expect and require advanced economic infrastructure. A modern society could not function without a network of paved roads, nor should it attempt to function without schools, universal healthcare, and other robust forms of economic infrastructure.
Even the Internet, while built of physical infrastructure, also includes volumes of economic infrastructure in the forms of protocols and software, much of it open source, which allows for interoperability that supports massive economies.
In seeking to shore up traditional infrastructure, it is important to do the same with these institutional, economic structures that are as important to the modern economy.