What is an "essential industry" or "frontline worker"?

During our state shutdown in spring of this year due to the COVID-19 epidemic, many Michiganders had to put their health and lives on the line working in industries that were critical to keeping our state running. Those frontline workers worked in a job that required them to work outside of their home, putting themselves at risk of exposure. Some examples of those frontline workers are those who staffed our grocery stores and restaurants, provided waste management services, manufactured Personal Protective Equipment (PPE), operated public transportation, or provided critical police or fire services.