What is National Health Insurance?

Japan's insurance system is a "universal health insurance system" where everyone is required to enroll in some form of medical insurance.
The National Health Insurance is a mutual aid system where all enrollees contribute to the National Health Insurance Tax to cover necessary medical expenses in preparation for illness or injury.
Except for those enrolled in their employer's health insurance (such as health insurance, seamen's insurance, mutual aid associations) and their dependents, those enrolled in the Late-Stage Elderly Medical Care system, and those receiving Public Assistance, everyone, regardless of age or occupation, is required to enroll in the National Health Insurance. (For example, self-employed individuals, those engaged in agriculture, part-time workers not enrolled in workplace health insurance, and those who have retired and left their workplace health insurance.)